This amazing cake, made with cocoa powder and espresso powder, is studded with dark chocolate chunks to make for a super chocolatey dessert.
Allow butter, eggs, and egg whites to stand at room temperature for 30 minutes. Meanwhile, grease and lightly flour two 6-cup fluted tube pans or one 10-inch fluted tube pan. In a medium bowl, stir together flour, cocoa powder, baking powder, baking soda, and salt. In a small bowl, dissolve espresso powder in hot water; stir in vanilla. Set aside.
In a large mixing bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Gradually add granulated sugar; beat until combined. Add sour cream; beat until combined. Add eggs and egg whites, 1 at a time, beating well after each. Alternately add flour mixture and espresso mixture to butter mixture, beating on low speed after each addition just until combined. Stir in chopped chocolate. Evenly pour batter into the prepared pan(s).
Bake in a 350 degree F oven for 35 to 40 minutes for small cakes or 50 to 55 minutes for large cake or until wooden toothpick inserted near center comes out clean. Cool for 15 minutes on wire rack. Remove from pan(s); cool completely on wire rack. Sprinkle with powdered sugar, if you like.