1. Preheat the oven to 350°F and line a baking sheet with parchment paper.

2. In a large bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth.

3. Add the egg and vanilla extract, then whisk until fully combined.

4. In a separate bowl, mix the flour, cocoa powder, hot cocoa mix, baking soda, and salt.

5. Add the dry ingredients to the wet ingredients and stir until a thick chocolate dough forms.

6. Fold in the chocolate chips until evenly distributed.

7. Scoop the dough into balls and place them on the prepared baking sheet, leaving space between each one.

8. Bake for 8 minutes, then carefully press mini marshmallows onto the tops of the cookies.

9. Return the cookies to the oven and bake for 2 to 3 more minutes, until the marshmallows puff and the cookie edges are set.

10. Let the cookies cool on the baking sheet for 10 minutes before transferring them to a rack. Sprinkle with flaky sea salt if desired.