Directions

1. Preheat oven to 350 degrees. Line standard muffin tins with paper liners. Coat an 8-inch square cake pan with cooking spray, line bottom with parchment, then spray parchment.
2. Whisk flour, baking powder, and salt in a large bowl. Place butter and chocolate in a heatproof mixer bowl set over a pot of simmering water, stirring until chocolate melts.
3. Attach bowl to mixer, add sugar, and whisk on medium-high speed until smooth, about 3 minutes. Beat in eggs, 1 at a time, then vanilla. Reduce speed to low, and add flour mixture.
4. Divide batter among muffin cups, filling each 2/3 full. Spread remaining batter in square pan. Bake until set but still soft, about 20 minutes. Let cool in tins and pan on wire racks.
5. Frost cupcakes with buttercream. Cut out 24 hearts from brownie in pan using a 1 1/2-inch heart-shaped cutter. Top each cupcake with a heart.