Preheat the oven to 350ºF.

In the bowl of a mixer, combine the butter and sugar and beat until light and fluffy, 1-2 minutes. Add in the eggs, one at a time, scraping down the sides of the mixer as needed. Beat in the vanilla.

In a separate bowl, combine the flour, cocoa powder, baking soda and salt. Add the dry ingredients to the wet ingredients and mix just until combined. Add in 3/4 cup of the marshmallows and the chocolate chips and mix until combined.

Scoop 1 tablespoon-sized balls of dough and roll into a ball. The dough will be stiff and sticky. Place on a parchment lined baking sheet, about 2 inches apart. With the remaining 1/4 cup mini marshmallows, press a few into the outside of each of the balls of dough.

Bake the cookies, one baking sheet at a time. Bake until the edges are set, but the center still looks a little bit wet, about 7 minutes. Remove from the oven and let the cookies sit on the baking sheets for a few more minutes before transferring to a baking rack or a sheet of waxed paper.