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

In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and brown sugar until light and fluffy (about 2-3 minutes).

Add the eggs one at a time, mixing well after each addition, followed by the vanilla extract until fully combined.

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

Gradually combine the dry ingredients with the wet ingredients using a spatula, mixing gently until just combined.

Fold in the chocolate chips, walnuts, and mini marshmallows until evenly distributed.

Using a cookie scoop, portion the dough onto baking sheets, ensuring they are spaced 2 inches apart. Bake for 10-12 minutes until edges are set.

After 8 minutes, gently press additional mini marshmallows into the tops of the cookies, then return to the oven for an additional 2-3 minutes.

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