Line a 9x9-inch or 9x13-inch pan with foil, leaving an overhang on the edges to create handles. Grease the foil with 1 tablespoon of butter.

In a large pot, melt the remaining 5 tablespoons of butter over medium-low heat. Add marshmallows and stir continuously until they are almost completely melted. Remove the pot from heat.

Stir in 2/3 cup of peanut butter and 1 teaspoon of vanilla extract until the mixture is smooth and well combined.

Fold in the Rice Krispies cereal, making sure all the cereal is evenly coated with the marshmallow mixture. Gently mix in the mini peanut butter cups.

Pour the mixture into the prepared pan. Press it gently into an even layer, being careful not to compact it too tightly.

In a microwave-safe bowl, combine chocolate chips and 3/4 cup of peanut butter. Microwave in 20-30 second intervals, stirring after each, until the mixture is smooth and fully melted.

Pour the melted chocolate mixture over the Rice Krispie layer. Use a spatula to spread it evenly across the top.

Allow the treats to cool at room temperature or place them in the refrigerator until the chocolate topping is firm. Use the foil handles to lift the entire block out of the pan. Cut into squares and enjoy!