In a large bowl whisk together the flour, icing sugar, baking powder and salt.
In a second medium bowl, whisk together the (room-temperature) milk, melted butter, vanilla and egg yolk until combined.
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.
Stir the wet ingredients into the dry ingredients until just combined (it’s ok if there are a few lumps). Stir in one-third of the beaten egg whites, then gently fold in the remaining egg whites taking care not to over mix.
Lightly spray the inside of four 3-inch-wide x 2½-inch-high ring molds with nonstick cooking spray.
Coat a large nonstick skillet with nonstick cooking spray and heat over medium-low heat.
Put the prepared ring molds in the middle of the skillet and fill each with 125 ml (½ 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.
Lightly spray the ring molds and coat the skillet with nonstick cooking spray and repeat the cooking method with the remaining batter.
Serve with butter and maple syrup. The pancakes should be eaten before they deflate.