Cream together the butter and both sugars until light and fluffy. Add in the vanilla and eggs, room temperature- remember? (Why room temp? Adding cold eggs to warm-ish butter causes the butter to seize and will change the texture of the cookie because the butter isn’t able to cream properly. whew.) Blend well. In a separate bowl, mix together the dry ingredients, including the cocoa mix.
Add the dry ingredients a little at a time to the wet ingredients until it’s all combined. Pour in the marshmallows and let mix for a minute or two so the marshmallows have a chance to get broken up a little bit. Big pieces of marshmallow tend to melt and make the cookies misshapen.
Scoop into 2-3 tablespoon balls and place on parchment paper-lined cookie sheets. Bake for about 8 minutes or until the tops of the cookies look set and no longer shiny. Remove from the oven and immediately place 4-5 marshmallows on top, pressing lightly into the cookie. Return the pan into the oven and let bake for 2 minutes more. The marshmallows should look puffed and just barely melted. Remove from oven and let cool for 5-10 minutes before transferring. Enjoy!