Combine the first five ingredients and cut together with a pastry cutter. Place in refrigerator.
Chop 2 oz chocolate bar into uniform pieces and put in a bowl with 3 tablespoon butter. Microwave on high for 50-60 seconds. Stir well to melt both chocolate and butter. Then add egg yolks and 2 tablespoons sugar. Mix well. Add mashed banana, heavy whipping cream, 1 tablespoon cocoa and 2 tablespoons all-purpose flour. Mix until well blended.
In another bowl, whip egg whites until stiff peaks form. Then add to the chocolate bowl dividing 2-3 times. Mix gently to blend well.
Pour the batter into a buttered and floured 6 inch cake pan. Take the crumble out of the refrigerator and sprinkle on top. Bake in preheated 350 degrees F oven for 30 minutes. Let cool on wire rack. Then refrigerate for 1-2 hours.