In a large bowl, beat the butter and coconut sugar together until fluffy. Add in maple syrup, egg, egg yolk, vanilla, and sea salt and beat again for about 1 minute, until combined.
Add in the flours and psyllium husk and slowly beat just until the dry ingredients are fully incorporated. Fold in chocolate chunks.
Wrap the bowl and chill dough for 24 hours.
After the dough has chilled, preheat your oven to 160ºC/325ºF and grease a 20-cm round baking tin.
Spoon half the cookie dough mixture into the tin and flatten it out with your fingers until it covers the bottom. Top with the chocolate hazelnut spread, smoothing it with a spoon until it covers the top. Add remaining dough and again flatten.
Bake for 15-20 minutes, until the edges are browned but the centre is still soft and gooey. Let cool for about 1 hour, if you can, so that it can set.
Serve as is or with some ice cream!
Notes
It'll still be delicious even if you don't let it set. I photographed and ate this batch almost straight after, and it was super gooey inside, just how I like it!
Recipe by Sprinkle of Green at https://www.sprinkleofgreen.com/deep-dish-nutella-filled-cookie-pie