Heat oven to 160C/140C fan/gas 3. Grease and line a 2lb loaf tin with baking parchment (allow it to come 2cm above top of tin). Mix the sugar, flour, bicarb, cocoa and chocolate in a large bowl.
Mash the bananas in a bowl and stir in the whole egg plus 2 yolks, followed by the oil and milk. Beat the egg whites until stiff. Quickly stir the wet banana mixture into the dry ingredients, stir in a quarter of the egg whites to loosen the mixture, then gently fold in the rest. Gently scrape into the tin and bake for 1 hr 10-15 mins, or until a skewer inserted comes out clean.
Cool in the tin on a wire rack. To make the icing, melt the chocolate and soured cream together in a heatproof bowl over a pan of barely simmering water. Chill in the fridge until spreadable. Remove cake from tin, roughly swirl icing over and scatter with the banana chips.