heat oven to 350 F and grease a 9 inch square baking dish
mix rhubarb sugar cornstarch lemon juice vanilla and salt then spread in dish and dot with butter
whisk flour baking powder baking soda and salt
whisk sugar egg buttermilk vanilla and melted butter then stir into dry ingredients just until combined
dollop batter evenly over rhubarb and gently spread to mostly cover.
Add streusel and bake
mix brown sugar flour cinnamon and butter until crumbly then stir in pecans if using and sprinkle over batter
bake until topping is golden and a toothpick in the cake portion comes out clean 40 to 50 minutes
cool 15 minutes before serving to let filling thicken.
Notes
1. Use frozen rhubarb without thawing and add 5 minutes bake time; substitute sour cream or plain Greek yogurt thinned with a splash of milk for buttermilk; for less sweet use 1/2 cup sugar in filling and 1/3 cup sugar in topping