Preheat oven to 350°F. Grease an 8-inch cast iron skillet or oven-safe skillet with butter.
In a large bowl, stir together the melted butter, brown sugar erythritol, egg, and vanilla extract until well combined and smooth.
Add the almond flour, baking soda, and sea salt. Mix until a thick, uniform cookie dough forms. Fold in the sugar-free dark chocolate chips, reserving a small handful for the top.
Press the dough evenly into the prepared skillet. Scatter the reserved chocolate chips across the surface and press them in lightly.
Bake for 20 to 24 minutes, until the edges are golden brown and set but the center still looks slightly soft. The center will firm up as it cools.
Let the skillet cookie cool for 10 minutes. Serve warm directly from the skillet, scooped with a spoon and topped with sugar-free vanilla ice cream if desired.
