Preheat oven to 350°F/180°C and line a baking tray with parchment paper (if you use a silpat baking mat they won't spread much, if at all).
Add the butter to a saucepan over medium heat and simmer, swirling the pan around frequently, until it begins to brown and bubble through (about 5 minutes). Transfer to a bowl and allow to come to room temperature, you can pop it in the fridge to speed up the process (just don't let it solidify ok?!).
Add almond flour, coconut flour, baking soda and xanthan gum to a medium bowl. Whisk until thoroughly combined and set aside.
Add the sweetener and molasses (optional) to the brown butter, mixing until very thoroughly combined (a couple minutes).
Add in vanilla extract and the egg, mixing well until just incorporated. Add your flour mix and beat it until very thoroughly combined.
Fold in the chocolate and scoop out your cookies onto a baking tray. I made mine fairly big (I find texture comes out ace this way), and got 8. But you could easily get a dozen if small is your game. Make sure to flatten them quite well, as I've found that these cookies don't tend to spread much (if at all!).
Bake for 9-10 minutes for smaller cookies and 12-13 minutes for the jumbo, turning the tray around 180° half way through.
An extra top tip: I found with some of the batches (excluding allulose which browns super quickly), that my cookies were coming out a bit on the paler side. Now I don't know if its because I have a convection oven for the first time (maybe you guys can chime in?), but what I did was bake them for about 9 minutes at 350F and then broil them for a minute or two until nice and golden.
Garnish with flaky sea salt (pretty please!) and allow the cookies to cool completely on the trays (erythritol in particular can take a few hours to crisp up, so don't despair!). Store in an airtight container for three to four days or freeze for up to 3 months.
Note that the shaped dough can be frozen for up to 3 months, and they can be baked straight from the freezer (adding 2-3 minutes more to the baking time).