Heat oven to 350 F and line a 12 cup muffin pan with liners
whisk flour baking powder and salt until evenly combined
cream butter and brown sugar until light and fluffy then beat in eggs one at a time and mix in vanilla
whisk milk rum and sour cream until smooth then add dry ingredients to the bowl in two additions alternating with the milk mixture mixing just until combined
divide batter evenly between liners filling each about two thirds full and bake until a toothpick comes out clean and tops spring back then cool 5 minutes in pan and cool completely on a rack before frosting.
Rum Brown Sugar Buttercream
Beat butter until smooth then beat in brown sugar until well blended
add powdered sugar gradually and mix on low then medium until fluffy
beat in cream rum vanilla and salt until smooth and pipeable adding a little more cream if too thick or more powdered sugar if too soft
pipe or spread onto fully cooled cupcakes and garnish if desired.
Notes
1. Use dark rum for deeper molasses notes or light rum for a milder finish. For alcohol free replace rum with 2 tbsp milk plus 1 tsp rum extract. Avoid overmixing once flour is added to keep a tender crumb