In a large bowl, combine butter and sugar with a mixer and beat it until light and fluffy, about 2 minutes. Add in one egg at a time and beat in the vanilla extract.
In a separate bowl, combine flour, cocoa powder, baking soda and salt. Add the dry ingredients to the wet ingredients and mix until just combined. Add in 3/4 cup marshmallows and chocolate chips and mix until combined.
Place one tablespoon-sized scoop of the dough and roll into a ball. Place on a parchment paper lined baking sheet at about 2 inches apart. With the remaining 1/4 cup marshmallows, press a few on the top of each ball of dough.
Bake the cookies one sheet at a time and bake for about 7 minutes or until the edges are set and the center still looks a little wet. Remove from oven and let leave the cookies on the sheet for a couple of minutes before transferring to a cooling rack or a sheet of wax paper.