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 combined. Be careful not to overmix. 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 until the mixture thickens, approximately 2 minutes.
05 - Drop large spoonfuls of the banana pudding mixture over the brownie batter. Use a knife or skewer to gently swirl through both layers, creating a marbled 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. Do not overbake to maintain a fudgy texture.
08 - Let the brownies cool completely in the pan before cutting into 12 squares. Garnish with fresh banana slices if desired. Serve chilled or at room temperature.