Preheat your oven to 160C Fan / 180C. Prepare a 9inch square baking tin with baking paper (leave enough overhang to lift out the bars later once cooked).

Add your melted butter and both sugars to a large bowl and mix together. Do this until totally combined - it will take a few minutes with a spatula by hand.

Add in your egg and vanilla extract and mix again until fully combined.

Add in your flour, xanthan gum, bicarbonate of soda, salt and cornflour and mix (I still do this by hand). It can take quite a bit of time to fully combine but you should be left with a yummy cookie dough!

Add in your chocolate chips and mix into the dough evenly. Leave a few chocolate chips out (20-30g) to pop in the top of the cookie bars.

Place your cookie dough into your prepared tin. I spoon mine in and push it to the edges making sure its an even layer without any gaps. (Get your hands in there if need be!)

Push some chocolate chips into the top of the cookie dough.

Place in your preheated oven for about 22-25 minutes until a skewer comes out fairly clean.

Leave to cool in the tin (you can lift them out to cool on a cooling rack after about 20 minutes if you want to).

Drizzle with a little extra melted chocolate (optionally) and then cut into squares. Enjoy!