Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

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

Beat in the eggs, one at a time, fully incorporating each before adding the next. Follow with the vanilla extract.

In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt.

Gradually add the dry ingredients into the wet mixture, mixing until just combined.

Gently fold in the chocolate chips, mini marshmallows, and chopped walnuts.

Scoop tablespoon-sized portions of the dough onto the prepared baking sheet, leaving about 2 inches between each cookie.

Bake in the preheated oven for 8-10 minutes, or until the edges are set but the centers remain slightly soft.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.