When the butter is mosty melted (about 80%), add 16 ounces marshmallows, and stir in.
Stir regularly until the marshmallows have melted.
Remove pot from heat.
Stir in cereal until combined.
Add remaining 10 ounces marshmallows, and fold in until incorporated.
Pour the cereal mixture into the prepared pan, and gently press the mixture into the prepared pan with a spatula.
Let cool to room temperature, and cut into bars to serve.
Video
Notes
Butter: I like to use salted butter in this recipe. The little bit of salt in the butter helps to balance out the sweetness. If you're using unsalted butter, add a pinch of salt.
Marshmallows: You can use 16 ounces of large marshmallows and 10 ounces of miniature marshmallows if you prefer.
Rice Krispies cereal: Or another brand of crispy rice cereal.