Old-Fashioned Rhubarb Upside-Down Bars are buttery vanilla cake bars baked over a sweet-tart rhubarb layer, then flipped so the fruit ends up on top. I like them because they’re simple, a little old-school, and the rhubarb gives you that sharp, bright flavor that keeps the whole thing from tasting flat.
The cake is soft and tender, and the topping sets into this glossy jammy layer once it cools. For me, this is the kind of dessert that works just as well on a weekend as it does on a spring holiday table.
I usually serve these slightly cool or at room temp so the rhubarb layer slices cleanly. They’re also really nice with whipped cream or a scoop of vanilla ice cream, especially if you want to lean into the “upside-down cake” feel.
Why You’ll Love This Recipe
Old-fashioned in the best way: It’s basically an upside-down cake, but in easy-to-serve bar form. The look is pretty without needing frosting or extra decorating.
Sweet-tart topping: Rhubarb stays punchy and bright here, which I think is exactly what you want with a buttery vanilla cake.
Good pan dessert: A 9×13 pan makes enough for a group, and the pieces stack and travel well for potlucks and bake sales.
Soft cake, jammy fruit: The contrast is the whole point—tender cake on the bottom and a sticky, glossy fruit layer on top.
Easy to tweak: Buttermilk, lemon zest, or a little cinnamon all work naturally with rhubarb, so you can nudge the flavor without changing the recipe.
Ingredients You’ll Need
Rhubarb Layer
- Rhubarb: Fresh rhubarb is great, but frozen works too—just thaw it and drain it well so the topping doesn’t turn watery.
- Granulated sugar: Rhubarb needs it. You can’t really cut it too much here or the layer will taste harsh and not “dessert-y.”
- Unsalted butter: A little butter in the fruit layer helps it turn glossy and rich once it bakes.
- Cornstarch: This is what helps the rhubarb juices thicken up so you get a sliceable topping instead of a puddle.
- Vanilla extract: Just a little makes the rhubarb taste more rounded and less sharp.
- Salt: Don’t skip the pinch. It makes the fruit layer taste more balanced.
Cake Batter
- All-purpose flour: I use regular flour here. Spoon and level if you tend to over-pack your measuring cup.
- Baking powder: This gives the bars their lift so the cake layer doesn’t bake up dense.
- Fine salt: It keeps the cake from tasting flat, especially with all that vanilla.
- Unsalted butter: Softened butter creams better and gives you that classic buttery vanilla-cake flavor.
- Granulated sugar: Keeps the crumb light and lets the rhubarb topping stay the star.
- Eggs: They hold the cake together and help it bake up tender instead of crumbly.
- Milk or buttermilk: Milk is totally fine, but buttermilk gives a nice little tang that matches the rhubarb really well.
- Vanilla extract: Use real vanilla if you can. This is a simple cake, so you taste it.
Recipe Instructions
- Heat the oven: Heat the oven to 350°F.
- Prep the pan: Grease and line a 9×13-inch baking pan with parchment paper, leaving an overhang so it’s easy to lift and flip later.
- Mix the rhubarb layer: In a bowl, mix the rhubarb with sugar, cornstarch, salt, vanilla, and melted butter.
- Add the rhubarb to the pan: Spread the rhubarb mixture evenly in the prepared pan and press it into an even layer.
- Mix the dry ingredients: Whisk the flour, baking powder, and salt together in a bowl.
- Cream the butter and sugar: Beat the softened butter and sugar until light and fluffy.
- Add the eggs and vanilla: Beat in the eggs, then mix in the vanilla until combined.
- Add the dry ingredients and milk: Add the dry ingredients alternating with the milk (or buttermilk), mixing just until combined and you don’t see dry flour.
- Top with batter: Dollop the batter over the rhubarb layer, then spread gently to cover the fruit as evenly as you can.
- Bake: Bake for about 35 to 45 minutes, until the top is golden and a toothpick inserted into the cake portion comes out clean.
- Cool briefly: Let the bars cool in the pan for 15 minutes.
- Invert the bars: Run a knife around the edges, then invert onto a cutting board or platter. Peel off the parchment and let cool before slicing.
Some of my tips!
This is the part that really makes a difference. If you only read one section, make it this one because this is where I share the little trick that helps the recipe turn out so well.
Recipe Tips & Tricks
Drain frozen rhubarb really well. Thaw it completely, then let it sit in a strainer for a bit. Extra liquid can make the topping too loose.
Press the fruit into an even layer. It helps the bars flip cleanly, and you don’t end up with thick spots that stay too wet.
Don’t overmix the batter once the flour goes in. Mix just until it comes together. Overmixing makes the cake layer tougher than it needs to be.
Be gentle when spreading the batter. The easiest way is to dollop it all over first, then spread lightly so you don’t drag the rhubarb around underneath.
Cool before slicing. The fruit layer sets as it cools, so you’ll get cleaner bars and a nicer top.
Make some touches!
There are plenty of easy ways to serve this depending on the occasion. Keep it simple, or add a few small touches to make it feel a little more special.
Serving Suggestions
- Whipped cream: A spoonful on the side is simple and classic. I like it best when the bars are completely cooled.
- Vanilla ice cream: If you want the full upside-down-cake vibe, this is it. Let the ice cream melt a little into the rhubarb topping.
- Strawberries: Fresh sliced strawberries are nice on the plate with rhubarb. It makes the whole dessert feel a little more “spring.”
- Coffee or tea: These are great with a mug of coffee or a strong black tea. I usually go this route if I’m serving them in the afternoon.
Save it for later
This section covers how to store leftovers and what parts of the recipe you can prepare ahead of time.
Storage and Make Ahead
Store the bars airtight at room temperature for up to 3 days. Keep them in a single layer if you can, or put parchment between layers so the tops stay neat.
If you want them to last longer, store them in the fridge for up to 5 days. I let chilled bars sit out for 15 to 20 minutes before serving so the cake softens up a bit.
You can also make this recipe ahead for gatherings. Bake the day before, keep it covered, and slice right before serving.
To freeze, freeze individual portions or freeze the whole slab well wrapped. Thaw in the fridge overnight before serving.
A few quick answers
This section covers a few extra details that may help before you head to the recipe card.
Recipe FAQs
Can I use frozen rhubarb?
Yes. Thaw it first and drain it well so you’re not adding a lot of extra liquid to the pan. If it still looks very wet, blot it a bit with paper towels.
How do I know when the bars are done?
The top should look golden, and a toothpick inserted into the cake portion should come out clean. Try to avoid poking straight into the rhubarb layer when you test.
Can I use buttermilk instead of milk?
Definitely. Buttermilk gives the cake a slightly tangier flavor, which works really well with rhubarb.
Any easy flavor add-ins that work well here?
Lemon zest in the batter is my favorite addition with rhubarb. A little cinnamon is nice too, especially if you want it to feel more like a coffee-cake bar.
Why did my rhubarb topping look a little loose after flipping?
It usually just needs more cooling time. The topping thickens as it cools, and it firms up even more after a short chill in the fridge.
You might also like
Before you go, take a look at a few other recipes below the recipe card. They use similar ingredients in a different way and are worth saving too.
MORE DINNER RECIPES TO TRY
Old-Fashioned Rhubarb Upside-Down Bars
INGREDIENTS
Rhubarb Layer
- 4 cups chopped rhubarb
- ¾ cup granulated sugar
- 2 tbsp unsalted butter melted
- 1 tbsp cornstarch
- 1 tsp vanilla extract
- pinch salt.
Cake Batter
- 2 cups all purpose flour
- 2 tsp baking powder
- ½ tsp fine salt
- ¾ cup unsalted butter softened
- 1 cup granulated sugar
- 2 large eggs
- 1 cup milk or buttermilk
- 2 tsp vanilla extract.
INSTRUCTIONS
Pan and Rhubarb Layer
- heat oven to 350 degrees F
- grease and line a 9x13 inch baking pan with parchment leaving overhang
- mix rhubarb with sugar cornstarch salt vanilla and melted butter
- spread rhubarb mixture evenly in pan and press into an even layer.
Batter and Bake
- whisk flour baking powder and salt
- cream butter and sugar until light then beat in eggs and vanilla
- add dry ingredients alternating with milk just until combined
- dollop batter over rhubarb then spread gently to cover
- bake until top is golden and a toothpick in cake portion comes out clean about 35 to 45 minutes
- cool in pan 15 minutes then run a knife around edges and invert onto a board or platter then peel parchment and cool before slicing.
