In a small bowl, mix flour, baking powder, baking soda and cocoa powder. Set aside.
In a separate medium bowl, cream butter and sugar.
Add egg yolks and vanilla to the butter-sugar mixture and beat well.
Thin the sour cream with the milk.
Alternately add the flour mixture and the sour cream mixture to the butter-sugar-egg yolk-vanilla mixture, beating well after each addition. Batter will be stiff, so scrape the bowl sides often.
In yet another bowl, whip the egg whites to form stiff peaks.
Gently fold the egg whites into the batter.
Line a muffin pan with cupcake liners and fill them 2/3rds full, using your spoon the level the tops. Bake and repeat as needed.
Bake at 350 degrees F for 15 to 18 minutes, or until cake springs back when lightly touched or toothpick inserted comes out clean.
Remove from pan when cool enough to do so and cool completely on wire racks. Frost and/or decorate as desired.