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.
Melt the white chocolate melts in the microwave in 30-second bursts. Stir after each round until smooth and completely melted. (Mine took about 2 minutes. Keep an eye on it because you do not want them to burn.)
Add the peanut butter to the melted white chocolate, and stir until smooth.
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.
Scoop the cookies onto the prepared baking sheets with a 1-tablespoon cookie scoop or a spoon. They will be sticky and messy, and this is OK.
Add extra toppings, if desired.
Refrigerate for 30-45 minutes, or until the white chocolate mixture is set.