Begin by lining a 9x13 inch pan with parchment paper and coating it with cooking spray. This will ensure easy removal of your treats once they're set.

Place a large pot over medium heat and melt the butter completely. Stir in the peanut butter until the mixture is smooth and well combined. Add the marshmallows to the pot, stirring frequently until they're almost completely melted. Continue stirring until all marshmallows are melted. Turn off the heat and stir in the vanilla extract.

Working quickly, pour the crispy rice cereal into the pot. Stir continuously until the cereal is evenly coated with the peanut butter marshmallow mixture. Transfer everything to your prepared pan and press it down firmly to create an even layer, using parchment paper and the back of a measuring cup.

For the chocolate topping, combine the chocolate chips and oil using a double boiler method (or the microwave), until the mixture is completely melted and smooth, stirring frequently. Pour the melted chocolate over the treats and spread it evenly across the surface.

Store in the refrigerator for 2 hours (or freezer for 45 minutes).