01 - Preheat oven to 350°F and line two baking sheets with parchment paper.
02 - In a medium bowl, whisk flour, cocoa powder, baking soda, and sea salt until evenly blended.
03 - In a large bowl, cream unsalted butter, light brown sugar, and granulated sugar with an electric mixer or whisk until light and fluffy.
04 - Add eggs one at a time, mixing well after each addition. Blend in vanilla extract until smooth.
05 - Gradually mix the dry ingredients into the wet mixture, stirring only until combined without overmixing.
06 - Gently fold in chocolate chips, chopped nuts, and 3/4 cup of mini marshmallows using a rubber spatula.
07 - Scoop rounded tablespoons of dough onto the prepared baking sheets, spacing cookies 2 inches apart.
08 - Press a few extra marshmallows and holiday sprinkles onto the tops of each dough mound for festive appearance.
09 - Bake for 10 to 12 minutes, until edges are set but the centers remain soft.
10 - Allow cookies to cool on baking sheets for 5 minutes before transferring to a wire rack to cool completely.