Melt the butter in a bowl in the microwave (in 10-15 second intervals). Crush the Digestive biscuits into small pieces using either a pestle and mortar, or by placing them in a plastic bag and hitting them with a rolling pin. You want a mixture of small pieces and a few crumbs.

Add the melted butter and sugars to a large mixing bowl and beat with an electric whisk (or wooden spoon) until combined.

Add the egg to the mixture and beat again until it combines and forms a runny batter.

Once the mixture starts to come together, add the chocolate chips and Digestive biscuits, reserving a few for placing on top of the cookies before baking.

Use your hands to bring the dough together. It should form a thick, sticky dough. Once it's completely mixed together, wrap the dough in clingfilm and refrigerate for at least one hour.

When ready to bake, preheat the oven to 180'C / Fan 170'C / Gas Mark 4. Line two baking trays with baking paper. Take the cookie dough from the fridge and divide it into 8 pieces.

Take a piece of the dough and flatten it into your hand. Place a marshmallow in the centre and roll the dough around it, using your hands to seal any gaps. Repeat until you have done this with all the dough.

Place 4-5 balls on a baking tray (you'll need plenty of space between them so I recommend doing this in batches. Keep any cookie dough balls still waiting to bake in the fridge until you're ready for them.)

Bake the cookies in the oven for 15-18 minutes. Remove from the oven and place a couple of extra chocolate chips and biscuit pieces carefully on top for decoration.

Allow the cookies to cool on the tray for 15-20 minutes before transferring to a baking tray to cool completely.