Set a rack in the middle of the oven and preheat to 350 degrees F. Lightly coat a 6-cup loaf pan with nonstick vegetable spray. Line the bottom with a rectangle of waxed paper. Sift the flour, baking soda, baking powder, and salt together into a bowl; repeat 2 times. In a mixing bowl, beat the butter and sugar together on high speed for 15 seconds, to combine. Add the eggs, 1 at a time, beating until each is incorporated. Continue beating until light and fluffy, about 6 more minutes.
In a bowl, mash the bananas until pasty. Stir in the sour cream and vanilla. With the mixer on its lowest setting, beat in 1/3 of the flour mixture. Beat in half of the banana mixture, then another 1/3 of the flour. Beat in the remaining banana mixture and the remaining flour. Spread the batter in the prepared pan and bake for 40 minutes or until the center springs back wen lightly pressed and a cake tester comes out clean. Set the pan on a rack to cool.
Turn the cooled cake out of the pan. With a serrated knife, trim off the end crusts and cut the loaf into 1/2-inch slices. Peel the bananas and cut them into 1/4-inch-thick rounds. Spread half the cake slices with the buttercream and layer on the banana slices. Place the remaining cake slices on top to form sandwiches. Cut the sandwiches in half and serve with ice cream and chocolate fudge sauce.
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