Preheat the oven to 180C / 350F / 160C fan forced. Line 2 cookie trays with baking paper.
Roll the marshmallows with a rolling pin until flat and about 3-4 mm thick. You can do these 4-5 at a time. Use the same cookie cutter you'll use for the cookies to cut them to the same size. Set aside.
Cream together the butter and sugar until pale and fluffy. This will take a few minutes, so while that’s happening, in a separate bowl mix the flour, salt and baking powder until it is all well mixed through. Add the egg and vanilla to the butter / sugar mix and beat until well combined.
Add the flour mix to the wet mix and beat until just combined. It will start to come together into a dryish, crumbly dough. This is good.
Pull it together into a ball with your hands, then roll the dough out on a very lightly floured surface to about 4mm thick. Use a 4cm fluted cookie cutter to cut rounds. Sit the rounds on the baking trays at least 1 inch apart. Bake for 7 minutes or until just golden at the edges.
Spread each biscuit with a little jam, then place one of the marshmallows on top. Press another biscuit down on top. Repeat with the rest of the biscuits.
Place the biscuits in the fridge for half an hour (being cold will help the chocolate to set quickly).
Melt the chocolate in 30 second increments in the microwave, stirring well between each until just melted. Line a large baking tray with baking paper.
Dip each biscuit into the chocolate and press down with your finger. Use a fork to turn it over, then lift it out. Use a small palette knife to scrape off the excess chocolate and just leave a thin layer.
Use a second fork and flip it from the first to the second. Use the palette knife again to scrape off the excess chocolate, then run the knife around the edge as well. Carefully set the biscuit down onto the baking paper on it’s flat side and repeat with the rest.