In a large saucepan over low heat, melt the 3 tablespoons of unsalted butter. Once melted, add the 10.5 ounces of miniature marshmallows and stir continuously until they are fully melted and combined with the butter, creating a gooey mixture.
Once the marshmallows are melted, remove the saucepan from the heat. Stir in 1 cup of creamy peanut butter until fully incorporated. The mixture should be smooth and creamy.
In a large mixing bowl, combine 4 cups of Rice Krispies cereal and 2 cups of crushed pretzel pieces. Pour the melted peanut butter and marshmallow mixture over the cereal and pretzels. With a wooden spoon, mix until everything is well-coated.
Add 1/2 cup of peanut butter chips and 2 cups of semisweet chocolate chips to the mixture. Gently fold them in until evenly distributed throughout the treat mixture.
Transfer the mixture into a greased 9x13 inch baking pan. Using a piece of wax paper or your hands, press the mixture evenly into the pan, ensuring it is tightly packed.
Allow the treats to cool at room temperature for at least 30 minutes. Once set, cut them into squares or rectangles, and they are ready to serve!
