Preheat your oven to 350°F (175°C).

Cream together the unsalted butter, granulated sugar, and brown sugar in a mixing bowl until smooth.

Beat in the large eggs one at a time, mixing thoroughly after each addition. Stir in the vanilla extract.

Combine the all-purpose flour, cocoa powder, baking soda, and salt in a separate bowl. Gradually mix this dry blend into the creamed mixture until just combined.

Fold in the semi-sweet chocolate chips, chopped walnuts, and mini marshmallows gently.

Drop rounded tablespoonfuls of the cookie dough onto ungreased baking sheets.

Bake in the preheated oven for 10 minutes.

Drizzle melted semi-sweet chocolate over the cooled cookies once they’ve set.