A buttery chocolate crust and a rum-scented, banana-imbued butterscotch blondie form two distinct layers that harmonize with every bite. The contrast between crunchy bottom and chewy topping is the whole point of the exercise.
04In a large skillet over medium heat, melt 12 tablespoons butter, then let it cook until the foam subsides and butter turns a deep nut brown, about 5 minutes. Cool.
05In a large bowl, whisk together bananas, eggs, dark brown sugar and rum. Whisk in brown butter. In a separate bowl, whisk together flour and fine salt. Fold into batter along with the toasted walnuts. Pour the mixture over the prepared crust and spread evenly. Sprinkle top with flaky salt if using.
06Transfer pan to oven and bake until the top is firm and a toothpick inserted in the center emerges with a few crumbs attached (or clean) but not wet, 45 to 55 minutes. Cool completely on a wire rack; cut into 24 bars.