01 - Preheat oven to 350°F and line two baking sheets with parchment paper.
02 - Whisk together flour, cocoa powder, baking soda, and salt in a medium bowl. Set aside.
03 - Beat unsalted butter, light brown sugar, and granulated sugar in a large bowl with an electric mixer until light and fluffy.
04 - Add eggs one at a time, mixing after each addition. Blend in vanilla extract until smooth.
05 - Gradually add the dry ingredient mixture to the wet mixture, mixing only until just combined.
06 - Gently fold in the chocolate chips, chopped walnuts or pecans, and 3/4 cup of the mini marshmallows.
07 - Portion rounded tablespoons of dough onto prepared baking sheets, spacing 2 inches apart.
08 - Lightly press extra marshmallows and holiday sprinkles onto each cookie.
09 - Bake for 10 to 12 minutes, until edges are set but centers remain soft.
10 - Let cookies cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.