Preheat oven to 425 F (218 C). Butter a 12-inch cast iron skillet.
Put the eggs, milk, and vanilla extract into a blender and blend until smooth (you can also do this in a bowl with a whisk). Sift in the flour, salt, and cinnamon; blend until smooth. Add the butter and blend.
Pour the batter into the prepared skillet. Sprinkle the blueberries on top. Bake at 425 for 15 minutes. Lower the oven temperature to 325 and bake for an additional 5-10 minutes, or until golden brown. Serve hot with lots of butter and a drizzle of maple syrup or a sprinkle of powdered sugar.