Melt butter in a large pot over low heat. Stir miniature marshmallows into melted butter; cook and stir until mostly melted, about 5 minutes. Add molasses, cinnamon, and nutmeg to the marshmallow mixture; continue cooking until the mixture begins to bubble, about 5 minutes more. Mix crushed peanuts into the mixture. Remove pot from heat.
Gently fold popcorn into the marshmallow mixture to coat, taking care to not crush kernels.
Working with buttered hands or a measuring cup sprayed with non-stick spray, form popcorn mixture into 12 balls and let rest on waxed paper until cool.