Preheat the oven to 425 degrees with a rack in the center position.Place the butter on a sheet pan and put the pan in the oven until the butter melts and begins to bubble, just a few minutes.While the butter is melting, whisk together the flour, salt and nutmeg in a large bowl.In a blender, combine the eggs, milk and vanilla and blend for 1 minute.Make a little hole in the center of the flour mixture and pour in the egg mixture, then whisk until combined. The batter will be really thin.Pour the batter onto the sheet pan (make sure the melted butter is evenly coating the pan first), sprinkle with cinnamon, then bake until the pancake is puffy and golden around the edges, about 15 to 20 minutes.When the pancake is done, remove it from the oven and sprinkle with desired amount of sugar.Cut into squares and serve immediately.