Add almond flour, coconut flour, xanthan gum, baking soda, cream of tartar and salt to a medium bowl. Whisk until thoroughly combined and set aside.
Cream butter in a large bowl with an electric mixer until softened, 1 minute. Add in sweetener and continue to cream until light and fluffy (about 8 minutes). Add in vanilla extract and egg, mixing until just incorporated. The mixture will appear slightly 'broken' (i.e. not thoroughly smooth). With your mixer on low, add in half of your flour mixture- mixing until just incorporated. Mix in the rest.
Transfer cookie dough to your skillet (or ramekins), cover with cling film and refrigerate for 30 minutes or up to three days.
Preheat oven to 375°F/190°C, sprinkle cinnamon 'sugar' mix on top and bake for 23-26 minutes (or until set and nice and golden!). I've noticed that baking time here varies a lot depending on your oven, skillet type (and size!) and altitude. So be sure to take a peak in the oven from minute 20 to ensure it doesn't over brown (a known 'issue' with allulose).
Allow to set for 10 minutes, top off with vanilla ice cream (non negotiable!) and enjoy.
Note that the cookie dough can be frozen for up to 3 months, and can be baked straight from the freezer (adding 5-7 minutes more to the baking time).
Perhaps surprisingly, getting the sweetener right for this keto snickerdoodle skillet cookie is even more important than the flours. And, without a doubt, you want to use either allulose (my favorite here because no aftertaste, zero GI and it actually browns!) or xylitol (ensure its not corn derived to avoid tummy troubles).Now, you can also use an erythritol sweetener (Lakanto’s golden is still my favorite) but do expect a bit of cooling aftertaste and a less gooey texture, particularly when cool (you can always rewarm it though!)How sweet?! In case you haven’t noticed, keto palates in regards to sweetness are all over the place (I feel mine fluctuates constantly still!). So I’m giving you guys a range of sweetener. If you use allulose I suggest using the larger amount as its 70% less sweet than the others.And if using xylitol, make sure to be careful if you have a pup around the house, as it’s highly toxic to the little guys! Either way, I prefer to use 1/3 cup of erythritol as opposed to the traditional 3/4 cup in cookies. This lessens the cooling effect if using erythritol and keeps them lightly sweetened. Oh! And it might just be me over-analyzing things (shocker!), but I found that erythritol version gave me no cooling aftertaste while the cookies were warm (so fresh from the oven or rewarmed slightly).