01 - Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving overhang on two 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 slightly pale.
03 - Stir in the cocoa powder, flour, and salt just until combined — do not overmix. Pour the brownie batter into the prepared pan and spread it evenly across the bottom.
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 the two layers together to create a marbled effect.
06 - Sprinkle the chopped vanilla wafer cookies evenly over the top. 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 the brownies to cool completely in the pan before cutting into squares. Garnish with fresh banana slices just before serving if desired.