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 granulated 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. Pour the batter into the prepared pan and spread it into an even layer.
03 - In a separate bowl, whisk together the whole milk, heavy cream, instant banana pudding mix, egg yolk, and mashed banana. Continue whisking for about 2 minutes until the mixture thickens to a pudding consistency.
04 - Drop large spoonfuls of the banana pudding mixture evenly over the brownie batter. Using a knife or skewer, gently drag through both layers in sweeping motions to create a marbled swirl effect. Avoid over-swirling to keep the layers distinct.
05 - Scatter the chopped vanilla wafer cookies evenly across the surface of the batter.
06 - Bake for 35 to 38 minutes, until the center is just set and a toothpick inserted near the edges comes out mostly clean with a few moist crumbs. The center may still have a slight jiggle but will firm as it cools.
07 - Let the brownies cool completely in the pan before cutting into 12 squares. Garnish with fresh banana slices if desired. For the cleanest cuts, chill before slicing.