Preheat the oven to 350°F (175°C). Butter a 12x9-inch baking pan. Prepare the brownie mix according to the package instructions and pour into the pan. Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with moist crumbs. Let cool completely, then cut into 3/4-inch slices.

Cut the miniature Reese's cups in half. Set aside for filling and garnish.

In a large bowl, beat the instant pudding mix with cold milk for 2 minutes until thickened. Add peanut butter and vanilla extract, mixing until smooth. Fold in 1 1/2 cups of Cool Whip.

In a 5-quart trifle dish, layer one-third of the brownies, one-third of the pudding mixture, and half of the Reese's cups. Repeat layers twice, finishing with a smooth layer of the pudding mixture.

Spread the remaining Cool Whip on top and garnish with halved Reese's cups. Chill in the refrigerator for at least 30 minutes before serving.