In a large mixing bowl, add butter, brown sugar, cocoa powder, coffee and baking soda. Beat well until combined. Dough may be stiff.
Add vanilla, egg whites and sour cream and beat well. Add flour and beat just until combined. Cover and refrigerate for 3 hours or overnight.
Preheat oven to 350 and line a baking sheet with parchment paper. Using a teaspoon scoop, scoop dough into your palm and roll into a ball. Dip into powdered sugar and generously coat the dough.
Place onto prepared baking sheet, 2" apart, and bake for 8-10 minutes or until edges are firm. Cool completely on a wire rack.