Preheat oven to 350 degrees. Line muffin tin with paper liners.
In a medium bowl, sift together cocoa, flour, baking powder, and salt; set aside. In a separate bowl mix together cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each, mix in vanilla. With mixer on low speed, add flour mixture in two batches, alternating with sour cream and beginning and ending with flour. Gently add in chocolate chips.
Pour batter into baking cups, filling each 3/4 full. Bake 20 to 25 minutes until a toothpick inserted in centers comes out clean.
Cool in pan 5 minutes; transfer to a wire rack to cool completely.