Preheat the oven to 375 degrees or 350 degrees convection bake.
Grease cookie sheet generously or use parchment paper.
In a large bowl, cream together butter, brown sugar and white sugar.
Beat in the egg, then stir in the vanilla. Do not over mix.
Pour the flour, cocoa, baking soda, salt, chocolate chips and marshmallows into the bowl, stir into the creamed mixture just until the flour disappears. If using a mixer, mix on low for a few seconds, or you may use a large spoon. This is a folding action, do not over mix! If the batter is a bit dry, fold in a tablespoon or two of milk.
Use a cookie or ice cream scoop and place the cookie dough balls onto the greased or lined sheet.
Flatten the cookies slightly with palm of hand.
Bake for 8 to 10 minutes or until the cookies are set on the outside.
Notes
-It is very important to generously grease the cookie sheet with a baking spray or to use a parchment paper liner. The marshmallows will melt and create a sticky residue which hardens and is difficult to clean. -This cookie should have the consistency of a chewy brownie, so baking just until the outer surface is cooked is best. These cookies are also really yummy refrigerated or frozen.