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1
Heat oven to 375 degrees.
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2
Line a 9-by-13-inch baking pan with parchment, and grease with butter.
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3
Melt 1 stick of butter in a saucepan over low heat or in microwave.
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4
Put the chocolate wafer cookies in the bowl of a food processor and process to make fine crumbs.
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5
Add the light brown sugar, melted butter and a pinch of salt.
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6
Process until the mixture is the consistency of damp sand.
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7
Dump the mixture into the pan and press it into an even layer.
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8
Bake for 7 to 10 minutes, or until the surface is firm.
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9
Remove the pan and set aside.
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10
Reduce oven heat to 350 degrees.
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11
In 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.
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12
Cool.
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13
In a large bowl, whisk together bananas, eggs, dark brown sugar and rum.
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14
Whisk in brown butter.
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15
In a separate bowl, whisk together flour and fine salt.
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16
Fold into batter along with the toasted walnuts.
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17
Pour the mixture over the prepared crust and spread evenly.
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18
Sprinkle top with flaky salt if using.
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19
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, 45 to 55 minutes.
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20
Cool completely on a wire rack; cut into 24 bars.
