Preheat the oven to 180°C (350°F) or 160°C for fan-forced ovens. Line two large baking trays with parchment paper and set aside.

In a medium mixing bowl, whisk together the all-purpose flour, Dutch-processed cocoa powder, baking soda, and salt. Ensure the mixture is evenly blended.

In a separate large mixing bowl, combine the melted butter with the light brown sugar and granulated sugar. Whisk thoroughly until smooth. Add the egg and vanilla extract, and whisk again until the mixture is well incorporated and glossy.

Gradually fold the dry ingredients into the wet mixture using a spatula. Stir just until combined, avoiding overmixing. Let the dough rest for 10 minutes to firm up slightly.

Scoop approximately two tablespoons of dough and roll into a ball. Create an indentation in the center and place six mini marshmallows inside. Carefully fold the dough around the marshmallows, ensuring they are fully enclosed. Roll the cookie ball gently to smooth the edges.

Place the cookie dough balls on the prepared baking trays, leaving about 2 inches of space between each one. Sprinkle with demerara sugar if desired. Bake in the preheated oven for 9–10 minutes.

For perfectly shaped cookies, use a slightly larger round cookie cutter to swirl around each cookie immediately after removing them from the oven. This will round the edges and thicken the cookies slightly.

Allow the cookies to cool on the baking trays for 10–15 minutes until firm enough to handle. Transfer them to a wire rack to cool completely before serving.