Make sure the butter is melted and set aside to cool a bit.
Preheat oven to 180C / 350F /160C fan forced. Line 2 large baking trays with baking paper.
In a medium bowl, whisk together the flour, cocoa, baking soda and salt. Set aside.
In a large mixing bowl, add the sugars to the butter and whisk (using a balloon whisk) until well combined.
Add the egg and vanilla and whisk until smooth.
Add the flour then fold it in with a spatula until just combined.
Let the dough sit for 10 minutes before proceeding (this makes it easier to handle and helps the cookies keep their shape).
Take roughly 2 tablespoons of the dough, roll it into a ball (or use a cookie scoop) then make a dent in the centre. Add 6 mini marshmallows to the middle then carefully pinch in the edges to cover completely.
Lightly roll the cookie ball between your hands to make it round and place on a baking tray, keeping them about 2 inches apart.
Sprinkle a little demerara sugar over the top, if using. Bake for 9-10 minutes.
Optional but recommended: As soon as you take the trays out of the oven, use a round cookie cutter, just slightly larger than the cookie, and swirl it around the cookie. This will make them perfectly round and also thicken them slightly.
Allow them to cool on the trays for 10-15 minutes until you can move them to a wire rack without them breaking, to cool completely. Once cool, they will firm up.
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For best results you should always weigh ingredients like flour and sugar. Kitchen scales are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).
All ovens vary – check your cookies at 8 minutes – they should look dry and ‘set’ but will still be very soft.
Dutch cocoa can be substituted with unsweetened cocoa.
Make sure the cookie dough is well sealed. If the marshmallow leaks out the side or bottom they can stick to the tray. If it's sealed inside, just the tops will crack allowing the marshmallow to peek out.
Make sure the butter has cooled to at least just warm before adding to the mixture.