Step 1

Place 12 tablespoons unsalted butter in the bowl of a stand mixer (or large bowl if using an electric hand mixer). Place 3 large eggs and 1 1/2 cups buttermilk on the counter. Let everything sit at room temperature until the butter is softened.

Step 2

About 30 minutes before the butter is ready, arrange a rack in the middle of the oven and heat the oven to 325℉. Line a 9x13-inch baking pan with parchment paper so that the paper hangs over two long sides to form a sling. Coat the paper and pan lightly with cooking spray.

Step 3

Peel and mash 4 medium (or 3 large) bananas in a medium bowl with a fork or potato masher until smooth with just a few small lumps (about 1 1/2 cups).

Step 4

Place 3 cups all-purpose flour, 1 1/2 teaspoons baking soda, and 1/2 teaspoon ground cinnamon in a medium bowl and whisk to combine.

Step 5

Add 3/4 cup granulated sugar, 3/4 cup packed brown sugar, and 1/2 teaspoon kosher salt to the bowl of butter. Beat with the paddle attachment on medium speed, using a flexible spatula to scrape down the sides of the bowl as needed, until fluffy, about 2 minutes total. Beat in the bananas and 2 teaspoons vanilla extract. The batter will look curdled. Beat in the eggs one at a time, beating well after each addition.

Step 6

Add 1/3 of the flour mixture and beat on the lowest speed until just combined. Beat in half of the buttermilk. Repeat beating in half of the remaining flour, the remaining buttermilk, and finally the remaining flour. Beat until the batter is smooth and combined, scraping down the sides as needed. Transfer to the pan and spread into an even layer.

Step 7

Bake until a tester inserted into the center comes out clean, 50 to 55 minutes. Transfer to a wire rack and let cool completely, about 2 hours. Meanwhile, make the frosting.

Step 8

Wash and dry the stand mixer and paddle attachment. Place 4 ounces cream cheese and 4 tablespoons unsalted butter in the bowl and let sit at room temperature until softened.

Step 9

Add 2 cups powdered sugar, 1 teaspoon vanilla extract, and 1/8 teaspoon kosher salt. Beat with the paddle attachment, starting on low speed for 30 seconds then increasing speed to medium-high, until well-combined, smooth, and fluffy, about 3 minutes total.

Step 10

Grasping the excess parchment, lift the cooled cake out of the pan and place on a cutting board. Spread the frosting over the cake in swoops. Cut into 24 pieces to serve.