Line two large baking sheets with parchment paper. Set aside. Combine the crispy rice cereal and marshmallows in a large bowl. Mix with a rubber spatula until combined. Set aside.

Pour the warm white chocolate-peanut butter mixture on top of the crispy rice cereal mixture. Stir until completely coated. Add any mix-ins, if desired, and stir until incorporated.

Once the avalanche cookies are chilled, transfer to an airtight container for storage. If layering the cookies, place a sheet of wax paper between the layers. Store in the refrigerator for up to 5 days.