Preparation

• Heat a large nonstick skillet over medium heat while you prepare the batter. Use 2 pans if you have them so you can cook all the pancakes at once.
  Place 4 large egg whites and 1/2 teaspoon cream of tartar in a stand mixer with the whisk attachment. Turn the mixer on to medium speed and slowly stream in 2 tablespoons of sugar, about 1 minute.
  Continue to beat until very foamy and increasing in volume, about 2 minutes more. Continue on high speed and beat until glossy and holds a stiff peak, and the bowl can be turned over without the meringue moving, 1 to 2 minutes.
  
  Whisk 2 large egg yolks, 2 tablespoons whole milk, and 1 teaspoon vanilla extract together in a large bowl. Whisk in the remaining 2 tablespoons sugar and 1/4 teaspoon salt.
  
  Sift all-purpose flour, cornstarch, and baking powder over the egg yolk mixture and whisk until combined as a creamy texture.
  
  Fold 1/3 of the egg white mixture into the yolk mixture with a spatula. Add the remaining egg white mixture and fold until just fully combined.
  
  Brush or wipe a layer of melted butter over the skillet.
  
  scoop 3 to 4 tablespoon portions of batter onto the heated skillet, spacing them evenly apart.
  
  Steaming:
  Cover the pan with a lid and cook for 4 to 5 minutes. Add 1 teaspoon of water to the pan to create steam, then cover again. After 5 to 6 minutes, gently flip the pancakes with a wide spatula. Add a little more water and continue cooking, covered, for another 4 to 5 minutes. When the bottoms look light golden to golden-brown, gently lift and flip with a thin spatula, reduce the heat and continue cooking for 3 to 4 minutes.
  Serve immediately, dusting with powdered sugar and topping with butter and maple syrup as desired. Repeat greasing the pan and cooking the remaining batter.