Preheat oven to 350 degrees. Line 12-cup standard muffin tin with paper liners.

In 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. Beat in eggs, one at a time, then beat in vanilla. Reduce speed to low. Add the flour mixture alternating with sour cream, starting and ending with the flour mixture.

Divide batter among cups, filling each 3/4 full. Bake until a toothpick inserted in center comes out clean, 20 to 25 minutes. Cool 5 minutes in pan, then transfer to a wire rack to cool completely. (Cupcakes can be stored overnight at room temperature, or frozen up to 2 months, in airtight containers.)

Transfer frosting to pastry bag fitted with a 1/2-inch star tip (such as Ateco #846). Pipe onto cupcakes and top with sprinkles.