Preheat oven to 180C. 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.
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.