Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and line a few baking trays (or however many you own) with baking paper
To make the cookie dough, mix the egg and the Nutella together
Add the flour and mix until you can't see any flour anymore
Divide the dough into two pieces, then split one of the pieces into 10
Roll each of the small pieces into balls using your hands, then flatten into discs and place onto the baking paper
Spoon about 1 tsp of Nutella on top of each cookie dough disc
Then split the other piece of cookies dough into 10, roll into balls and flatten into discs as before, then place each disc over the first disc, covering the Nutella filling and pressing down around the edges to seal the filling in
Bake for 8-10 minutes, allow to cool on the tray for at least 5 minutes, then move to a cooling rack to cool fully
Store in an airtight container in a cool place and eat within 3 days