In a large mixing bowl, combine the melted butter, sugars, egg, egg yolk and vanilla extract. Mix well until the ingredients are fully incorporated.
Add the gluten-free flour, almond flour, salt, bicarbonate of soda and nutmeg to the bowl. Mix the dry ingredients into the wet ingredients until a dough forms.
Stir the 100g of chocolate chunks into the dough until evenly distributed. Set aside the remaining chopped chocolate for later use.
Place the dough in the refrigerator for 8 hours or overnight if desired.
Pre-heat the oven to 190°C /170°C fan-assisted /325F / gas mark 5 . Line a large baking tray with parchment paper.
Remove the dough from the refrigerator and portion it into rough oblong shapes, each weighing approximately 40g.
Place them onto the prepared baking tray, spaced apart. You will have to bake the cookies in batches, possibly 6 in each batch.
Bake the cookies in the pre-heated oven for 10 minutes, or until the edges are lightly golden.
Once baked, remove the tray from the oven. While the cookies are still warm, use a large round cookie cutter to swirl around the cookies to gently coax them into perfect circles. Press some of the extra chocolate chunks into the tops of the cookies, and sprinkle a small amount of flaked sea salt over each cookie.
Allow the cookies to cool on the tray for 5 minutes. Then transfer them to a wire rack to finish cooling completely.
Continue with baking another batch of cookies until you have used up all the dough.