Preheat oven to 350 degrees. Cut 3 pieces parchment paper to fit bottom of three 8-inch cake pans. Grease each pan and add parchment rounds.
Combine sugar, cake flour, cornstarch, baking soda and salt in large bowl. In separate bowl, combine cocoa powder and coffee. Add yogurt and combine well.
In another bowl, with hand mixer, whip egg whites into soft peaks. With same hand mixer, mix melted butter into dry mixture and then add cocoa mixture until batter is smooth. Fold in egg whites.
Pour batter evenly into prepared 8-inch cake pans. Bake for 25-35 minutes, or until toothpick inserted in middle of each cake comes out clean. Remove cakes from oven and place on cooling rack for 10 minutes.