01 - Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper, allowing the edges to overhang 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.
03 - Fold in the cocoa powder, all-purpose flour, and salt, stirring gently just until incorporated. Avoid overmixing to keep the brownies tender. Pour the batter into the prepared pan and spread it into an even layer.
04 - 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.
05 - Drop large spoonfuls of the banana pudding mixture over the brownie batter. Using a knife or skewer, gently drag through the layers in a figure-eight motion to create a marbled swirl effect.
06 - Scatter the chopped vanilla wafer cookies evenly across the surface of the batter.
07 - 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.
08 - Allow the brownies to cool completely in the pan before cutting into 12 squares. Garnish with fresh banana slices if desired and serve at room temperature or chilled.