Preheat oven to 350°F.
Line the bottom of two 12-cup muffin tins with paper liners.
Combine the flour, cocoa, baking soda, and salt. Set aside.
Cream butter and sugar until light and fluffy in separate bowl using an electric mixer.
Add egg and vanilla extract. Beat until fully incorporated.
Alternate adding dry ingredients and sour cream to butter/sugar mixture in three parts.
Add milk and mix until uniform.
Divide batter into the muffin tins, filling each 3/4 full.
Bake for 15 minutes or until a toothpick inserted in middle comes out clean.
Cool in the muffin tins for a short time. Then transfer to a wire cooling rack.