Step 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.

Step 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.

Step 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.

Step 4

Divide batter among muffin cups, filling each 2/3 full. Spread remaining batter in square pan. Bake until set but still soft, about 18 minutes. Let cool in tins and pan on wire racks.

Step 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.