Preheat the oven to 350°F. Line a 9x13 baking dish with parchment paper, set aside.

In a large bowl, mix the brownie mix with the ingredients listed on the back of the box. Spread about ⅔ of the batter into the prepared baking dish.

Place 15 Reese cups on top of the batter, and press them in slightly.

Gently spread the remaining batter on top of the cups, ensuring they are completely covered with batter.

Bake for 25-30 minutes until a toothpick inserted into the center of the brownie comes out mostly clean; a few moist crumbs are okay, but any wet batter and it needs to bake longer.

Place on a wire rack to cool completely.

Once the brownies have cooled, make the frosting by placing the peanut butter and butter into a large bowl with an electric hand mixer or stand mixer with the paddle attachment. Cream until smooth.

Add the powdered sugar in a little at a time until fully mixed in; scrape the sides as needed. The mixture will be thick.

Add the heavy cream and vanilla, and stir it until combined. Place the mixer on medium-high speed and whip for 2 minutes until light and fluffy.

Using the parchment paper to help, lift the brownies out of the baking dish.

Spread the frosting evenly over the brownies. Serve immediately or add more fun toppings if desired.