1. Arrange a rack in the middle of the oven and heat to 325 F. Line a 9-by-13-inch baking pan with parchment paper so that the paper hangs over two long sides. Coat the paper and pan lightly with butter or cooking spray.

2. Place the butter, granulated sugar and brown sugar in the bowl of a stand mixer fitted with the paddle attachment. (Alternatively, use an electric hand mixer and large bowl.) Beat on medium speed until fluffy, about 2 minutes. Reduce the speed to low and beat in the eggs one at a time. Continue to beat until the batter is silky and lightened. Add the bananas and vanilla, and beat until creamy and fully combined.

3. Whisk the flour, cocoa powder, baking soda, and salt together in a medium bowl. With the mixer on low speed, slowly add the flour mixture and milk in alternating additions until just combined. Fold in the chopped chocolate by hand with a rubber spatula. Transfer the batter to the prepared pan and spread into an even layer.

4. Bake until a toothpick comes out clean, 50 to 55 minutes. Transfer to a wire rack and let cool completely. Meanwhile, make the frosting.

5. Wipe out the bowl of the stand mixer. Add the peanut butter, cream cheese, butter, powdered sugar and vanilla, and beat on low speed until combined. Increase the speed to medium and beat until fluffy and lightened in color, about 3 minutes. Spread over the cooled cake in swoops. Sprinkle with flaky salt, and cut into 24 pieces to serve.

Recipe notes: Leftovers can be wrapped in plastic wrap and stored at room temperature for up to three days.

(Grace Elkus is a contributor for TheKitchn.com, a nationally known blog for people who love food and home cooking. Submit any comments or questions to editorial@thekitchn.com.)