Combine the flours, cocoa, sugar, baking soda and salt and stir to mix well.
Add the banana, soured milk (milk with dash of lemon juice... or buttermilk will work), egg whites and vanilla extract, and stir to mix well.
Coat a 9x13 pan with nonstick cooking spray; spread the batter evenly in the pan.
Bake at 350 for about 35 minutes, or just until a wooden toothpick inserted in the center of the cake comes out clean; cool the cake to room temperature.
To make the glaze, combine the glaze ingredients in a small bowl.
Microwave for 35 seconds or until runny.
Drizzle the glaze over the cake and let harden before cutting in squares and serving.