Preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a large mixing bowl, cream together the butter, brown sugar, and granulated sugar until light and fluffy.

Add the eggs one at a time and then the vanilla extract, beating until smooth.

In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gradually add to the wet ingredients.

Fold in the chocolate chips and mini marshmallows, if using.

Scoop rounded tablespoons of dough onto the baking sheets, chill for 30 minutes.

Bake for 9 to 10 minutes until edges are set but centers are soft. Cool for 5 minutes before transferring to a wire rack.

For the frosting, beat the butter until creamy, then mix in the marshmallow fluff and gradually add powdered sugar and vanilla.

Once cookies are cooled, frost each with the marshmallow frosting. Optionally drizzle with melted chocolate or sprinkle with crushed candy canes.