To make these cookies, start by preparing the mini marshmallows. Add the mini marshmallows to an airtight container and place them in the freezer for a few hours before baking.
When you're ready to make the cookies, preheat your oven to 350 F.
Next, add the flour, baking soda, and sea salt to a medium-sized bowl. Whisk the ingredients together, then set it aside.
Then, in an even larger bowl, whisk together the softened butter and sugars with a hand mixer. Once they're incorporated well, add in the peanut butter, egg, and vanilla extract, and mix again until they're fully combined.
Remove the marshmallows from the freezer and add them to the batter, folding them in quickly before they defrost.
Then, add the flour mixture into the bowl with the wet ingredients and mix until a batter is formed.
Once the batter is done, set it aside and line a baking sheet with parchment paper.
Use a cookie scoop to scoop the dough out into balls and line them up on the baking sheet. I use a large cookie scoop, which makes each ball about 2-3 tablespoons worth of dough. If you don't have a cookie scoop, you can also use a spoon!
Space the balls of dough out about 1-2 inches apart to make room for when they spread.
Bake the cookies for 10-11 minutes. Once they're done, remove them from the oven and allow them to cool for another 10 minutes - they will continue to bake while cooling!
Once the cookies are cool, serve them and enjoy! Store any leftovers in an airtight container, in the fridge, for up to one week.