Peel, core, and slice the apple 1/8 inch thick. Melt 3 tablespoons of the butter in a 9-inch ovenproof crêpe pan or skillet over medium to medium-high heat. Add the apple, 1 tablespoon of the granulated sugar, and the cinnamon and toss well. Sauté the apple for 5 to 7 minutes until tender, tossing occasionally. Spread the apple out in the pan and keep hot over low heat.
Meanwhile, melt the remaining tablespoon of butter in the microwave. In a medium bowl, whisk together the milk, eggs, melted butter, and vanilla. In a small bowl, combine the flour, the remaining tablespoon of granulated sugar, and the salt. Whisk the flour mixture into the milk mixture, whisking until there are no lumps. Pour the batter over the cooked apple in the hot pan and immediately place in the oven.
Bake for 12 to 15 minutes, until the pancake is puffed and the edges start to get crispy and browned. Dust lightly with sifted confectioners’ sugar, cut in wedges, and serve hot with maple syrup.