In a large, deep saucepan, melt butter over low heat, about 4 minutes. Add mini marshmallows and cook, stirring, until melted, 6 to 8 minutes.
Add chocolate and cook, stirring, until melted, 2 to 4 minutes. Remove from heat. Quickly fold in rice cereal and peanut-butter chips until evenly distributed.
Coat a 1/4-cup dry measuring cup with nonstick cooking spray; scoop mixture onto waxed paper, forming clusters. Let set, about 10 minutes.
Store in an airtight container for up to several days.