Special equipment: four 3-inch-wide-by-2 1/2-inch-high ring molds
1. Whisk together the flour, confectioners' sugar, baking powder and salt in a large bowl.
2. Whisk together the milk, melted butter, vanilla and egg yolk in a medium bowl until combined.
3. Beat the egg whites and cream of tartar in another large bowl with an electric mixer on medium-high speed until stiff peaks form, about 2 minutes.
4. Stir the milk mixture into the flour mixture until just combined (it's OK if there are a few lumps). Stir one-third of the beaten egg whites into the flour-milk mixture. Then gently fold in the remaining egg whites until just combined (take care not to overmix).
5. Lightly spray the inside of four 3-inch-wide-by-2 1/2-inch-high ring molds with nonstick cooking spray.
6. Heat a large nonstick skillet over medium-low heat. Coat with nonstick cooking spray. Put the prepared ring molds in the middle of the skillet and fill each with 1/2 cup of batter (it should fill each ring mold about halfway). Cover the skillet with the lid and cook until the batter rises to the tops of the ring molds and is golden on the bottom, about 5 minutes. Release the bottom of the pancakes with a spatula. Grasp the sides of the ring molds with tongs to stabilize them and then carefully flip. Cover and cook until golden on the other side, about 5 minutes more. Transfer to a plate and remove the mold. Serve with butter and maple syrup. The pancakes should be eaten before they deflate.
7. Lightly spray the ring molds and coat the skillet with nonstick cooking spray and repeat the cooking method with the remaining batter.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)