Preheat oven to 350°F (175°C). Grease a 9×13-inch baking pan. Prepare the brownie mix according to package instructions and spread into the pan. Bake for 20–25 minutes until a toothpick comes out clean. Let cool completely, then cut into ¾-inch cubes.
Make the Pudding Mixture:
In a large mixing bowl, combine the vanilla pudding mixes, cold milk, peanut butter, and vanilla extract. Whisk for about 2 minutes until thickened. Gently fold in 1½ cups of Cool Whip until fully incorporated.
In a clear glass trifle dish or large serving bowl, begin with a layer of brownie cubes. Sprinkle half of the mini Reese’s halves on top. Spread one-third of the pudding mixture over the candy layer.
Add another layer of brownies, the remaining Reese’s pieces, and another third of the pudding mix. Top with the final layer of brownies and remaining pudding mixture.
Spread the remaining Cool Whip evenly over the top. Garnish with additional Reese’s halves if desired. Cover and refrigerate for at least 2 hours before serving to let the flavors meld together.