Step 1: Make the Cookie Dough

1. Preheat oven to 350°F (175°C).
2. In a mixing bowl, combine flour, cocoa powder, and salt. Mix well.
3. In another bowl, cream softened butter and sugar until light and fluffy.
4. Add the egg and vanilla extract to the butter mixture, mixing thoroughly.
5. Gradually add the dry ingredients to the wet mixture, stirring gently to form the dough.

Step 2: Add Chocolate and Marshmallow
6. Fold in the chocolate chips.
7. Swirl in marshmallow fluff or gently fold in mini marshmallows to create a marbled effect.

Step 3: Shape and Bake
8. Using a tablespoon or cookie scoop, drop spoonfuls of dough onto a parchment-lined baking sheet.
9. Bake for 10-12 minutes, or until edges are set and centers are slightly soft.
10. Let cookies cool on the baking sheet for a few minutes, then transfer to a wire rack.