Begin by melting the butter and allowing it to cool slightly.

Preheat your oven to 180°C (350°F) or 160°C if using a fan-forced setting. Line two baking trays with parchment paper.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.

In a large bowl, combine the melted butter with both sugars and whisk until the mixture is well blended.

Add the egg and vanilla extract to the sugar mixture and whisk until smooth and fully incorporated.

Gradually fold in the dry ingredients using a spatula, stirring until just combined.

Let the cookie dough rest for 10 minutes, allowing it to firm up slightly for easier handling.

Scoop out about 2 tablespoons of dough per cookie. Roll each portion into a ball and press a small indentation in the center. Place about 6 mini marshmallows into the indentation, then carefully pinch the edges to seal the marshmallow inside.

Gently roll the filled cookie dough ball between your hands to smooth it out, and place on the prepared baking trays, ensuring there is about 2 inches of space between each cookie.

If desired, sprinkle a light dusting of demerara sugar on top of each cookie before baking.

Bake for 9-10 minutes. For perfectly round cookies, use a slightly larger cookie cutter to swirl around the edges immediately after removing them from the oven, which also thickens them slightly.

Let the cookies cool on the baking trays for 10-15 minutes before transferring them to a wire rack to cool completely. As they cool, they will firm up.