1-First Step: Prepare the Brownies Start by mixing 18 ounces of brownie mix with the ingredients listed on the package, such as eggs, oil, and water, as directed. Pour the batter into a greased 9×13 baking dish and bake at 350°F for about 30 minutes, or until a toothpick inserted in the center comes out mostly clean. Let the brownies cool completely on a wire rack, then refrigerate for at least one hour to firm up, which makes layering easier and prevents the marshmallow from melting too quickly.

2-Second Step: Add the Marshmallow Layer Once the brownies are chilled, take them out of the fridge and evenly spread 7 ounces of marshmallow creme over the top using a spatula. Make sure to cover the entire surface for that gooey effect. Return the dish to the refrigerator for another 30 minutes to let the marshmallow set, which helps it hold when you add the next layer and keeps the bars from getting too messy.

3-Third Step: Top with Peanut Butter Cups After the marshmallow has firmed up, sprinkle the halved 25 mini peanut butter cups over the top, pressing them gently into the marshmallow so they stick. This step adds the signature peanut butter flavor and creates those fun pockets of chocolate in your Reeses Marshmallow Brownie Bars. If you are adapting for dietary needs, like a nut-free version, swap these for chopped candy bars as needed.

4-Fourth Step: Make the Crispy Topping In a microwave-safe bowl, combine 12 ounces of semi-sweet chocolate chips with 1 cup of creamy peanut butter. Heat in 30-second intervals, stirring after each one until the mixture is smooth and fully melted, which should take about 1-2 minutes total. Then, stir in 3 cups of crispy rice cereal until every piece is coated, creating a crunchy layer that contrasts the soft parts below.

5-Fifth Step: Apply the Topping and Chill Carefully spread the chocolate-peanut butter crispy mixture over the peanut butter cup layer, making sure it covers everything evenly. Use a spatula to smooth it out without disturbing the layers underneath. Pop the dish back into the refrigerator for at least one hour to let everything set firmly, which ensures the bars cut neatly and hold their shape for serving.

6-Final Step: Cut and Serve Once chilled, remove the dish from the fridge and cut into bars using a sharp knife. For the best results, let them sit at room temperature for about 10 minutes before serving to soften slightly. These Reeses Marshmallow Brownie Bars are perfect for sharing at gatherings, and you can store leftovers as outlined later. Total prep time is 15 minutes, cook time is 30 minutes, and chilling adds at least 2 hours, so plan ahead for the full experience.