3/4 cup unsweetened cocoa powder
3/4 cup all-purpose flour
1/2 teaspoon baking powder
1/4 teaspoon salt
3/4 cup (1 1/2 sticks) unsalted butter, room temperature
1 cup sugar
3 large eggs
1 teaspoon vanilla extract
1/2 cup sour cream
Preheat oven to 350 degrees. Line 12-cup standard muffin tin with paper liners.
Into a medium bowl, sift together cocoa, flour, baking powder, and salt; set aside. In a mixing bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each, then beat in vanilla. With mixer on low speed, add flour mixture in two batches, alternating with sour cream and beginning and ending with flour.
Pour batter into cups, filling each 3/4 full. Bake until a toothpick inserted in centers comes out clean, 20 to 25 minutes.
Cool in pan 5 minutes; transfer to a wire rack to cool completely, then spread with chocolate frosting using a piping bag or small offset spatula. Decorate with sprinkles, if desired.