Tip the flour, salt, and baking soda into a medium bowl and whisk to combine. Set aside.
In another large bowl, cream together the butter, coconut sugar, and caster sugar until fluffy and pale. It’s best to use an electric hand mixer for this, but you can use a wooden spoon.
Add the egg and egg yolk and beat to fully combine. Mix in the vanilla.
Tip in the dry ingredients and mix just until all the flour disappears. Switch to a rubber spatula and fold in the chocolate. Cover the bowl with a piece of foil and refrigerate for a minimum of 3 hours. For best results, chill overnight.
Once the dough has chilled, preheat the oven to 180 C (350 F) and line a baking sheet with a piece of non-stick paper or a silicone mat.
Divide the dough into 9-10 portions - each portion should be 2 tablespoons - and roll into balls. Place the dough balls on the baking tray with a good amount of space between them because they will spread. You may have to use two trays. Gently press the balls down to create thick discs.
Bake the cookies for 13-15 minutes, or until they’re dry on top and golden around the edges and on the bottom. It’s fine if they’re still a bit gooey in the centre - they’ll continue to cook as they cool. Leave them on the tray to cool for 5 minutes and firm up, then remove them to a wire rack to finish cooling completely.