The four bananas in this extra-fancy banana bread ensure a light texture and pronounced flavor. A tangy cream cheese-sour cream frosting complements the sweetness. Both bread and filling can be made ahead and held, or assembled right before serving.
For the banana bread: Position an oven rack in the middle of the oven and preheat to 350 degrees F. Line an 18-by-13-inch rimmed baking sheet with parchment paper, allowing for a 1-inch overhang at each long end. Spray the parchment with nonstick cooking spray and set aside.
In a medium bowl, whisk together the flour, baking soda, cinnamon and salt. In the bowl of a stand mixer fitted with the paddle attachment, mash the bananas on medium speed until soft. Turn the speed to medium-low and add the granulated sugar, oil, vanilla and eggs and mix until the bananas are pureed and the mixture is smooth. Turn the speed to low and add the flour mixture to the banana mixture in 2 additions, mixing well between them. Mix until just combined.
Spread the batter out in an even layer in the prepared baking sheet using a spatula, and tap the pan against the countertop once to remove air bubbles. Bake until a toothpick inserted into the center comes out clean and the bread is golden and pulls away from the pan, 22 to 25 minutes. Let cool in the pan 15 minutes. Remove the bread using the parchment overhang to a baking rack to cool completely, 30 minutes.
For the frosting: Meanwhile, in the bowl of a stand mixer fitted with the paddle attachment, beat the cream cheese on medium speed until softened, 1 minute. Add the confectioners' sugar in 2 batches, beating on low speed until combined after each addition, 1 minute. Turn the speed to medium and add the sour cream and heavy cream and beat until smooth, silky and well-combined. Transfer to a resealable plastic bag and chill.
For assembling the napoleons: Transfer the bread with its parchment to a large cutting board. Using a ruler to get a straight edge, trim 1/4-inch from all the edges. Cut the bread into thirty-five 3-by-1 1/2-inch rectangles (cut every 3 inches along the length and 1 1/2 inches along the width.). Line the rectangles up vertically on a cutting board. Cut the corner off of the bag of frosting, and pipe a zigzag of frosting over each of the pieces. Stack 2 banana bread and frosting layers upon each other. Repeat with the remaining layers of bread and frosting to make seventeen 2-layer napoleons (you will have one unpartnered piece of cake left over). Top each napoleon with a banana chip, if using.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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