Butter a 9"x13" Pyrex or other baking pan.
Measure out cereal into a large, heat-proof bowl.
6 cups Rice Krispies cereal
In a medium to large sized pot, melt the peanut butter and butter together over low heat.
½ cup smooth peanut butter, 3 tablespoons (43 grams or 1.5 ounces) unsalted butter
Add the marshmallows and stir until melted.
10 ounces marshmallows, homemade or store-bought
Quickly stir the peanut butter-marshmallow mixture into the cereal.
Once it has been mostly mixed, add the rainbow chips and quickly mix.
½ cup rainbow chips or sprinkles
Transfer the mixture into the buttered pan, and use the spatula or your hands to spread the mixture evenly into the pan, pushing down on it gently.
Let it cool for at least 30 minutes before cutting.
Store at room temperature, covered with plastic wrap, for one week.