01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving enough overhang on the sides to lift the finished brownies out easily.
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 slightly glossy. Fold in the cocoa powder, all-purpose flour, and salt, stirring just until no dry streaks remain. 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 surface. Using a knife or skewer, gently drag through both layers in sweeping motions to create a marbled swirl effect. Avoid over-swirling to keep distinct pockets of each flavor.
05 - Scatter the chopped vanilla wafer cookies evenly across the top of the swirled batter, pressing them lightly into the surface so they adhere during baking.
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. The center may still have a slight jiggle but will firm up as it cools.
07 - Allow the brownies to cool completely in the pan on a wire rack. Once fully cooled, use the parchment overhang to lift the slab out and cut into 12 even squares. Garnish with fresh banana slices if desired. Serve chilled or at room temperature.