Peel four of the bananas and mash with a fork in a medium sized mixing bowl.
Mix the eggs, sugar, and vegetable oil in a second mixing bowl until frothy. Stirring constantly, add the crème fraîche, then flour, mashed bananas, and baking powder. Pour the batter into a rectangular cake pan or casserole dish and bake at 350 °F for 17 minutes.
Mix the butter, cream cheese, powdered sugar, and vanilla extract.
When the cake has cooled, spread the buttercream evenly over the top and then garnish with slices from the remaining banana. As a finishing touch, add a top layer of chocolate sprinkles.
