Prepare the Pan: Lightly grease a 9x13 inch baking dish with cooking spray or butter and set aside.
Melt the Marshmallows: In a large saucepan, melt the butter over low heat. Add the marshmallows and stir continuously until completely melted and smooth.
Add Peanut Butter and Vanilla: Remove from heat, then stir in the peanut butter and vanilla extract until well combined.
Combine with Cereal: Add the Rice Krispies cereal to the melted mixture. Stir gently until the cereal is fully coated with the marshmallow-peanut butter mix.
Press into the Pan: Pour the mixture into the prepared baking dish. Using a spatula or your hands (greased), firmly press the mixture evenly into the dish.
Melt the Chocolate: In a microwave-safe bowl, combine the dark chocolate and milk chocolate chips. Microwave in 30-second intervals, stirring in between, until the chocolate is completely melted and smooth.
Top the Treats: Pour the melted chocolate over the pressed Rice Krispie mixture, spreading evenly with a spatula. Optionally, sprinkle a pinch of sea salt over the top for added flavor.
Chill and Cut: Refrigerate the dish for about 30 minutes to allow the chocolate to set. Once firm, cut into squares or bars.
Serve and Enjoy: Enjoy these delicious treats as a snack or dessert!