Preheat oven to 350 F. Grease a 10-inch cast-iron skillet with 1 tablespoon of butter.
In a bowl, beat the butter, brown sugar, erythritol, and granulated erythritol with an electric mixer until light and fluffy.
Add the eggs, one at a time, beating well after each addition. Add the vanilla and salt and mix to combine.
Add the baking soda and almond flour and mix until just incorporated. Add the chocolate chips and stir to combine using a rubber spatula.
Pat the cookie dough into the skillet, and bake until golden brown and the middle is almost set about 25 minutes. Let cook for 10 minutes, cut into wedges, and serve.