Preheat oven to 350 degrees. Line a large baking sheet with parchment.
In a bowl, cream the butter, brown sugar, and sugar until soft and fluffy. Whip in the eggs, one at a time, then the vanilla. Add the flour, cocoa powder, black cocoa powder, baking soda, and salt. Mix until just combined.
Scoop heaping 3 tablespoon sized mounds of dough and flatten into a disc. Add 8 mini marshmallows to the center, then press the dough together and roll into a ball. Set onto the prepared baking sheet, fitting no more than 5-6 cookies to allow space for spread.
Bake the cookies 8 minutes, then remove the pan from the oven and tap firmly against the counter a few times. Return to the oven for an additional 3-4 minutes. Remove and let cool on the baking sheet.