- 1 stick unsalted butter
- 100 grams chocolate wafer cookies (to make 1 1/2 cups crumbs)
- 2 tbsp light brown sugar (about 1/4 cup)
- 1/4 tsp fine sea salt, plus a pinch
- 1 ripe banana, mashed
- 1 large egg
- 1 1/4 cups dark brown sugar
- 1 tablespoon dark rum
- 1/2 cup all-purpose flour
- 1/4 cup toasted walnuts, chopped
- Flaky sea salt, for sprinkling (optional)
Heat oven to 375 degrees. Line an 8-by-8-inch baking pan with parchment, and grease with butter.
Melt 3 tbsp butter in a saucepan over low heat or in microwave. Put the chocolate wafer cookies in the bowl of a food processor and process to make fine crumbs. Add the light brown sugar, melted butter and a pinch of salt. Process until the mixture is the consistency of damp sand. Dump the mixture into the pan and press it into an even layer. Bake for 7 to 10 minutes, or until the surface is firm. Remove the pan and set aside.
Reduce oven heat to 350 degrees. In a large skillet over medium heat, melt 5 tablespoons butter, then let it cook until the foam subsides and butter turns a deep nut brown, about 5 minutes. Transfer to a bowl to cool.
In 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.
Transfer 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, 35 to 45 minutes. Cool completely on a wire rack; cut into 12 bars.