01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving slight overhang on the sides for easy removal.
02 - In a large bowl, whisk together the melted butter and sugar until well combined. Add the eggs and vanilla extract, whisking until the mixture is smooth and glossy. Fold in the cocoa powder, flour, and salt, stirring just until incorporated — do not overmix.
03 - In a separate bowl, combine the whole milk, heavy cream, instant banana pudding mix, egg yolk, and mashed banana. Whisk vigorously for about 2 minutes until the mixture thickens to a pudding consistency.
04 - Pour the brownie batter into the prepared pan and spread evenly. Drop large spoonfuls of the banana pudding mixture over the batter. Using a knife or skewer, gently drag through both layers to create a marbled swirl pattern.
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.
07 - Allow the brownies to cool completely in the pan on a wire rack before cutting into squares. Garnish with fresh banana slices if desired, then serve chilled or at room temperature.