In mixer, combine butter, granulated sugar, brown sugar, eggs and vanilla.
In separate bowl, combine flour, baking soda and salt.
Stir cocoa powder into butter mixture. Mix in flour mixture.
Stir in marshmallows and chocolate chunks.
Chill dough for 15-30 minutes.
Scoop dough in about 2 tablespoons onto cookie sheet lined with parchment paper or silpat, 2 inches apart. Bake about 10 minutes. They'll look gooey and undercooked. Let cool.