Preheat oven to 350. Grease 10"-inch tube pan or bundt pan.
Chop 3 chocolate squares. In a large mixing bowl with electric mixer, beat the butter and granulated sugar until fluffy. Add in eggs, one by one, beating well after each egg until well blended. Add in bananas and sour cream; blend well.
In other bowl combine the flour, dry pudding mix, baking soda and salt; stir. Add this dry mixture in to the banana mixture; beat until well incorporated. Add in chopped chocolate; stir well. Pour the dough in to the greased pan.
Bake 45 minutes. Cool in pan 10 minutes. Turn the pan upside down on to a wire rack; slowly remove the pan. Cool cake completely. Place on a serving plate.
Put the remaining chocolate square in small microwave-safe bowl and melt in microwave. Stir until smooth and fully melted. Add in cream cheese and stir until completely combined and melted. Add in powdered sugar and stir the mixture until is well incorporated. Stir in milk. Drizzle over sides of the cake and the center of the cake. Store the cake in fridge.