In large mixing bowl, cream cold butter cubes and sugar for at least 4 minutes, scraping the bowl halfway through mixing.
Add eggs one at a time, mixing well after each addition. Add in vanilla.
Stir in flour, cocoa, baking powder, and salt. Stir in chocolate chips, marshmallows, and walnuts. Try to keep the marshmallows toward the center so there is a gooey marshmallow center.
Place onto cookie sheet lined with a silpat or parchment paper.