Preheat and Prep:
Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.

Mix Dry Ingredients:
In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt.

Cream Butter and Sugars:
In a large bowl, cream softened butter, granulated sugar, and brown sugar until light and fluffy (2–3 minutes).

Add Wet Ingredients:
Beat in the egg and vanilla until fully combined.

Combine Dry with Wet:
Gradually add dry ingredients to wet ingredients in 2–3 batches. Mix until just combined.

Fold in Chocolate Chips:
Gently fold in the semi-sweet chocolate chips.

Add Marshmallows and Fluff:
Fold in mini marshmallows and optional marshmallow fluff. Avoid overmixing to maintain swirl patterns.

Scoop and Bake:
Scoop dough onto prepared baking sheets, spacing 2 inches apart. Bake for 8–10 minutes, until edges are set and centers are soft.

Cool and Serve:
Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.