1. Prepare Oven and Pan:

• Preheat the oven to 350°F (175°C) or according to the temperature specified on your brownie mix box.
• Line a 9×13 inch baking dish with parchment paper, leaving an overhang on the longer sides to use as handles later. Grease the parchment paper lightly. Set aside.

2. Mix Brownie Batter:

• In a large bowl, prepare the brownie mix batter according to the specific instructions listed on the back of the box, using the required eggs, oil, and water. Mix until just combined – do not overmix.

3. Layer Batter and Reese’s Cups:

• Spread about two-thirds (⅔) of the prepared brownie batter evenly into the bottom of the parchment-lined baking dish.
• Unwrap the 15 full-sized Reese’s Peanut Butter Cups. Place them evenly on top of the batter layer, pressing them in just slightly.
• Gently spoon or pour the remaining one-third (⅓) of the brownie batter over the top of the Reese’s cups. Use a spatula to carefully spread the batter, ensuring all the Reese’s cups are completely covered.

4. Bake the Brownies:

• Place the baking dish in the preheated oven.
• Bake for 25-30 minutes (or according to your brownie mix package directions, potentially adding a few extra minutes due to the filling).
• The brownies are done when a toothpick inserted into the brownie portion (avoiding a Reese’s cup) near the center comes out with a few moist crumbs attached. If it comes out with wet batter, bake for a few minutes longer.

5. Cool Brownies Completely:

• Remove the pan from the oven and place it on a wire rack.
• Let the brownies cool completely in the pan before frosting. This is crucial, as warm brownies will melt the frosting. Cooling can take 2-3 hours.

6. Make the Peanut Butter Frosting:

• Once the brownies are cool, make the frosting. Ensure your butter is properly softened.
• Place the softened butter and creamy peanut butter into a large bowl. Using an electric hand mixer or a stand mixer fitted with the paddle attachment, cream them together on medium-high speed until smooth and combined.
• Reduce the mixer speed to low. Gradually add the sifted powdered sugar, about one cup at a time, until it’s mostly incorporated. Scrape the sides of the bowl as needed. The mixture will be thick.
• Add the heavy cream and vanilla extract. Start mixing on low speed until combined, then increase the speed to medium-high.
• Whip the frosting for 2 minutes until it becomes light, fluffy, and spreadable. If it seems too thick, add another splash of cream; if too thin, add a bit more sifted powdered sugar.

7. Frost and Serve:

• Using the parchment paper overhangs, carefully lift the completely cooled brownie block out of the baking dish and place it on a cutting board or serving platter.
• Spread the prepared peanut butter frosting evenly over the top of the brownies.
• Slice into squares or bars. Serve immediately, or store covered.