Preheat oven to 350°F.
Line two 12 count-cupcake pans with paper liners.
In a medium bowl, mix together flour, cocoa powder, baking soda, and salt with a whisk. Set aside.
Using an electric mixer, beat butter on medium-high setting using a paddle attachment until creamy. Gradually add sugar and beat on high until well blended. Reduce speed to medium, add eggs, one at a time, and then vanilla, until blended.
Reduce speed to low, then mix in half of dry ingredients slowly, then sour cream, and then the remaining dry ingredients until well blended.
Scoop batter into lined baking cups, filling each about 2/3 full. Bake for 20 minutes, or until a toothpick inserted into center of a cupcake comes out cleanly. Cool cupcakes in pan for 15 minutes, then remove to a wire rack to cool completely.