Preheat oven to 350 degrees F.
Line muffin pan with 12 cupcake wrappers.
In a large bowl, combine cocoa powder, flour, baking soda and salt. Mix well and set aside.
In a separate bowl, combine egg, egg yolk and sugars. Mix well by hand to combine. Stir in milk and sour cream, stirring well to combine. Add in coffee and butter, mixing well to combine. Mix with dry ingredients until well combined.
Fill liners with cupcake batter, approximately 3/4 of the way full.
Bake for 15-20 minutes or until cupcakes bounce back when lightly pressed and toothpick inserted into center comes out clean. Remove from oven and allow to cool for at least 30 minutes prior to frosting.