Brownie Heart Cupcake

Ingredients:

Vegetable oil, cooking spray
3 cups all-purpose flour
1 1/2 teaspoons baking powder
1 1/2 teaspoons coarse salt
3 sticks unsalted butter, cut into pieces
12 ounces unsweetened chocolate, coarsely chopped
3 cups sugar
6 large eggs, room temperature
1 tablespoon pure vanilla extract

Directions:

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.
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.
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.
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.
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.

Basic Buttercream frosting:

Ingredients
12 ounces (3 sticks) unsalted butter, softened
1 pound confectioners' sugar, sifted
1/2 teaspoon pure vanilla extract

Directions
Beat butter with a mixer on medium-high speed until pale and creamy, about 2 minutes.
Reduce speed to medium. Add sugar, 1/2 cup at a time, beating after each addition, about 5 minutes. (After every 2 additions, increase speed to high, and beat for 10 seconds, then reduce speed to medium-high). Add vanilla, and beat until buttercream is smooth. Use immediately, or cover and refrigerate for up to 3 days. (Bring to room temperature, and beat on low speed until smooth before using.)