Add almond flour, cinnamon, xanthan gum, baking soda, and salt to a medium bowl. Whisk until thoroughly combined and set aside.
Cream butter in a large bowl with an electric mixer, 2-3 minutes. Add in sweetener and continue to beat until thoroughly light and fluffy and much of the sweetener has dissolved.
Add in 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.
Wrap cereal dough with cling film (saran wrap) and refrigerate for at least an hour and up to 3 days.
Preheat oven to 350°F/180°C.
Roll out the dough between two pieces of parchment paper until nice and thin. Using a ruler, cut dough lengthwise and then crosswise into squares. Prick each piece with a small fork (optional).
Transfer parchment paper with the keto cinnamon toast crunch to a baking sheet or tray, and place in the freezer for 10 minutes prior to baking. The shaped dough can be frozen for up to 3 months, and baked straight from the freezer (adding 2-3 minutes more to the baking time).
Bake for 8-12 minutes, until fully golden (depending on size and thickness). Keep an eye out for them, and note that if you like them crisp you'll want to push the baking time as much as possible (until deep golden).
Brush with melted butter and sprinkle with cinnamon 'sugar'. You can also do this prior to baking for a more 'crusty' result, both work well just depending on your preference so feel free to experiment.
Allow to cool for ten minutes before transferring to a cooling rack. Allow to cool completely, as they'll continue to crunch up (this may take a few hours for the sweetener to harden up again!).
Store in an airtight container for up to 5 days.
*Coconut butter can be substituted if dairy free or paleo. Just note that your cereal will be less airy (and therefore crunchy). Having said that, still much crunchier than our previous version!