Combine the butter and sugars in a stand mixer with a paddle attachment, cream together on medium-high for 2 to 3 minutes. Scrape the sides of the bowl, add the egg and vanilla, and beat for 7 to 8 minutes.
Reduce the mixer speed to low and add the flour, baking powder, baking soda, and salt. Mix just until the dough comes together - no longer than a minute.
On low speed, paddle in the frosted flakes and mini m&m's until they’re incorporated.
Divide dough into 1/4 c. balls, stuff with 2 or 3 mini marshmallows, and place balls on a parchment-lined cookie sheet. Refrigerate for at least an hour, or place in freezer for 30-45 minutes. (They must be chilled!)
Heat the oven to 365°f
Re-arrange the dough balls minimum 4 inches apart on parchment- or silpat-lined cookie sheet and bake for 11-16 minutes (check the cookies at 11 minutes, mine took 12!) The cookies will puff, crackle, and spread.
Cool the cookies completely on cookie sheets before transferring to a plate. Store in airtight containe, if you have any left to store.