01 - Preheat oven to 350°F. Line an 8×8-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.
02 - In a large bowl, whisk melted butter and sugar until well combined. Add eggs and vanilla extract, whisking until the mixture is smooth and slightly glossy. Fold in cocoa powder, flour, and salt, stirring just until no dry streaks remain. Pour the batter into the prepared pan and spread into an even layer.
03 - In a separate bowl, whisk together whole milk, heavy cream, instant banana pudding mix, egg yolk, and mashed banana until the mixture thickens, about 2 minutes.
04 - Drop large spoonfuls of the banana pudding mixture over the brownie batter. Using a knife or skewer, gently drag through both layers in sweeping motions to create a marbled effect. Avoid over-mixing to maintain distinct swirls.
05 - Scatter the chopped vanilla wafer cookies evenly across the surface of the batter.
06 - Bake for 35 to 38 minutes, or until the center is just set and a toothpick inserted near the edges comes out mostly clean with a few moist crumbs. Do not overbake.
07 - Allow to cool completely in the pan before cutting into squares. Garnish with fresh banana slices just before serving if desired.