01 - Preheat the 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.
03 - Fold in the cocoa powder, 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. Using a knife or skewer, gently swirl through the layers to create a marbled effect.
06 - Scatter the chopped vanilla wafer cookies evenly over the surface. Bake for 35 to 38 minutes, or until the center is just set and a toothpick inserted near the edges comes out mostly clean.
07 - Allow to cool completely in the pan before cutting into squares. Garnish with fresh banana slices if desired, and serve chilled or at room temperature.