Preheat oven to 425 Degrees
Add cold butter, cut in tablespoon chunks, to an electric mixer and beat on LOW until butter is somewhat softened
Add sugar and cocoa powder and beat on LOW until well combined
Mix in one egg at a time, scraping bowl's sides and bottom as needed
Add in dry ingredients, pulse mixer or mix on LOW until well combined.
Fold in marshmallows and chunks of chocolate baking bar
Bake in oven for 7 minutes (cookies will be done when they start to crack on top)
Let cookies rest on hot baking sheets for 30 or so minutes to finish baking!