Heat oven to 350. Coat 8-inch square baking pan with cooking spray; sprinkle with flour, tapping out excess.
Whisk flour, baking soda and salt in medium bowl. Whisk bananas, 1/2 cup sour cream, egg and 1/2 teaspoon vanilla in separate medium bowl.
Beat butter, sugar and brown sugar in large bowlat medium-high speed 3 minutes or until fluffy, scraping bowl occasionally. Add banana mixture, beat 1 minute or until smooth. At low speed, slowly add flour mixture, scraping bowl after each addition. Stir in 4 oz. chocolate. Spoon and spread batter in pan.
Bake 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool completely on wire rack.
Place 3 oz. chocolate in medium microwave-safe bowl. Microwave on high 30 seconds, stir. Continue to microwave at 10-second intervals until almost melted. Stir until melted and smooth; let stand until room temperature. Whisk in all remaining frosting ingredients until smooth. Spread over top of cake.