Preheat the oven according to the brownie mix package instructions. Prepare the brownie batter using the additional eggs, oil, and water specified on the package. Bake as directed.

Remove the baked brownies from the oven and allow them to cool completely. Cut or crumble them into small, bite-sized pieces and set aside.

In a large mixing bowl, combine the cold milk and both packages of instant vanilla pudding mix. Whisk for approximately 2 minutes, or until smooth and beginning to thicken. Refrigerate while preparing the remaining ingredients.

In a separate mixing bowl, combine the creamy peanut butter and vanilla extract. Stir until smooth, then gently fold in about half of one container of thawed Cool Whip to create a lighter, more spreadable peanut butter mixture.

Unwrap the miniature Reese's peanut butter cups and chop most of them into small pieces. Reserve a handful of whole or halved peanut butter cups for decorating the finished dessert.

Place half of the crumbled brownies in the bottom of a large glass trifle bowl or deep serving dish, spreading them into an even layer.

Spoon half of the peanut butter mixture over the brownies. Sprinkle with a layer of chopped Reese's peanut butter cups, then spread half of the prepared vanilla pudding over the candy.

Repeat the layers using the remaining brownies, peanut butter mixture, chopped Reese's, and vanilla pudding.

Spread the remaining Cool Whip evenly over the top of the dessert. Decorate with the reserved Reese's peanut butter cups and a few brownie crumbs, if desired.

Cover the trifle bowl and refrigerate for at least 2 hours to allow the layers to set and the flavors to blend.

Serve chilled, using a large spoon to scoop through all the layers so each serving contains brownies, peanut butter filling, pudding, candy, and whipped topping.