Whisk almond flour, coconut flour, baking powder, baking soda, xanthan gum, if using, and salt together in bowl. Whisk erythritol, melted butter, almond butter, eggs, and vanilla in large bowl until well combined. Stir flour mixture into erythritol mixture until just combined, then fold in chocolate chips. Cover and let dough rest at room temperature for 30 minutes.

Adjust oven rack to middle position and heat oven to 350 degrees. Line 2 rimmed baking sheets with parchment paper. Divide dough into 16 portions, about 2 tablespoons each, and roll into balls using your hands. Arrange 14 balls on 1 prepared sheet spaced 2 inches apart. Using damp fingers, flatten dough balls until 3 inches in diameter. Freeze dough disks until firm, about 1 hour, then transfer to zipper-lock bag and return to freezer.

Meanwhile, arrange remaining 2 dough balls on second sheet and press into 3-inch disks. Bake until light golden brown and edges have begun to set but centers are still soft, 8 to 10 minutes, rotating sheet halfway through baking. Let cookies cool completely on sheet. Serve.