Preheat oven to 180 C / Gas 4. Grease and flour a 23x33cm or similar sized tin. Sieve together the flour, cocoa, bicarbonate of soda and baking powder. Set aside.
Beat egg yolks until thick and lemon coloured. Gradually beat in sugar. Blend in chocolate and almonds.
In a large bowl, cream together the butter, margarine and sugar until light and fluffy. Add the eggs one at a time, beating well with each addition, then add the vanilla. Add the flour mixture alternately with the milk. Mix to combine.
Beat egg whites until stiff peaks form. Fold whites into chocolate batter. Spoon batter into an ungreased 25cm cake tin.
Pour batter into the prepared tin. Bake for 30 minutes in the preheated oven or until a skewer inserted into the cake comes out clean. Allow to cool.
Bake at 180 degrees C for 1 hour, or until cake springs back when lightly touched. Remove from oven, turn it upside down on a cooling rack and cool for about 40 minutes before removing from tin.
Spread whipped cream over cooled cake and place sliced bananas on top. Store in refrigerator.