1. Prepare Your Pan: Line a 9×13 inch baking pan with parchment paper, leaving an overhang on the sides. This will make it much easier to lift the entire batch out of the pan for cutting later. Lightly grease the parchment paper if desired, though it’s often not necessary with the high fat content of the base.

2. Melt Butter and Peanut Butter: In a large microwave-safe bowl, combine the unsalted butter and creamy peanut butter. Microwave in 30-second intervals, stirring well after each, until completely melted and smooth. Alternatively, you can do this over low heat on the stovetop in a large saucepan, stirring constantly.

3. Create the Peanut Butter Base: Remove the melted mixture from the heat (or microwave). Stir in the powdered sugar and vanilla extract until well combined and smooth. The mixture should be thick but still pliable.

4. Add Crispy Rice Cereal: Gently fold in the crispy rice cereal until it is evenly coated with the peanut butter mixture. Make sure not to crush the cereal too much; you want to maintain its crunch.

5. Press into Pan: Transfer the peanut butter cereal mixture into the prepared 9×13 inch pan. Using the back of a spoon or your hands (lightly greased if it’s too sticky), press the mixture down firmly and evenly into the pan. Ensure it’s packed tightly to form a solid base.

6. Prepare the Chocolate Topping: In a separate microwave-safe bowl, melt the semi-sweet chocolate chips. Microwave in 30-second intervals, stirring until smooth and fully melted. Stir in the mini marshmallows until they are just coated and slightly softened by the warm chocolate, but not completely melted.

7. Spread Chocolate Topping: Pour the chocolate and marshmallow mixture evenly over the peanut butter base in the pan. Use an offset spatula or the back of a spoon to spread it smoothly to the edges.

8. Chill and Cut: Place the pan in the refrigerator for at least 1 hour, or until the chocolate topping is firm and set. Once firm, use the parchment paper overhang to lift the entire slab out of the pan. Place it on a cutting board and cut into squares using a sharp knife. For clean cuts, you can wipe the knife with a warm, damp cloth between cuts.