Melt the unsalted butter and set aside to cool a bit. You need the butter only slightly warm.
Preheat the oven to 350 degrees F and line the baking tray with baking /parchment paper.
Mix flour, cocoa, baking soda, and salt in a medium bowl. Set aside.
In another large mixing bowl, add the sugars to the butter and whisk until well combined. Add the egg and vanilla and whisk the mixture until smooth.
Add the dry ingredients and fold them with a spatula until just combined.
Let the cookie dough rest for 10 minutes.
Take about two tablespoons of the cookie dough and roll it into a ball between your palms.
Make a cavity in the center and add six mini marshmallows to the middle.
Pinch the edges of the cookie dough and seal it properly to cover the marshmallows completely.
Lightly roll the cookie dough balls between your hands to make them round. Place the balls on a baking tray, about 2 inches apart.
Sprinkle the cookie dough balls with a little demerara sugar over the top if using. Bake the cookies for 9-10 minutes.
Optional: Place a round cookie cutter, slightly larger than the cookie, on top of the cookie and swirl it around every piece of cookie. This will make them perfectly round and also thicken them slightly.
Allow the cookies to cool on the trays for 10-15 minutes until they harden enough to move them to a wire rack without breaking.
The cookies will firm up once they cool.