• Preheat your oven to 180°C (160°C fan) gas mark 6 and line a couple of baking sheets with baking parchment or greaseproof paper.
• Cream the butter, light muscovado sugar and vanilla in your mixer with the paddle beater on medium speed until pale and fluffy. Reduce the speed and beat in the egg until combined.
• Add the flour and mix on slow until combined and you have a sticky dough.
• Drop in the chocolate chunks and mix on slow just to distribute a bit.
• With your hands take out a generous ball of the dough with couple/few chocolate chunks in and roll them into a large walnut size ball. It’s pretty sticky and a good tip is to dampen your hands first, this helps to roll it without sticking too much.
• Put 8 balls on each large baking tray, they will flatten and spread into a large disc so allow plenty of space.
• Bake for around 12 to 15 minutes, until the cookies are lightly golden around the edges but are still a bit squidgy in the centre. Allow to cool on the tray.

To make your sandwich cookies:

• Preheat a grill to a high heat and line a baking sheet with parchment.
• Put 8 of the cookies on the tray with the flat side facing up and top with a large marshmallow, (if they are the really big ones, snip them in half) or a few small, just to fit within the edge of the cookies allowing for some spreading out.
• Grill until the marshmallow begins to brown and melt. Alternatively if you have one you can do this with a blowtorch.
• It’s easier to pop your Nutella in a piping bag to warm it and get it piped neatly, then you can pipe a lovely slick of Nutella all over your other cookies, or alternatively, spread Nutella on the cookies and sandwich on top of the melted marshmallow layer.
• Enjoy Marshmallow sandwich cookies and prepare to get messy!