Preheat oven to 425°F and coat a 12-inch cast iron skillet with butter (about two tablespoons).
In a large bowl, beat together the eggs, milk and vanilla extract.
Sift in the flour, xantham gum, salt and cinnamon. Mix until all the dry ingredients are incorporated. Stir in the melted butter. Batter might be slightly lumpy, which is fine.
Pour batter into the prepared skillet. Sprinkle the blueberries on top of the batter, keeping them within two inches of the edge of the pan.
Bake for 15 minutes at 425°F then lower the heat to 325°F and bake for 10 more minutes. Dust with powdered sugar before serving, serve immediately.