Separate the eggs while they’re still cold, placing the whites in a medium bowl and the yolks in a larger bowl. Cover both bowls and let them sit at room temperature for at least 30 minutes before you start preparing the cake.
Pre heat oven to 350 degrees F. Spray a 9 x 13 inch baking pan with non-stick cooking spray and set aside.
Whisk together all purpose flour, baking powder and salt together in a small bowl and set aside.
Using a handheld electric mixer, beat the egg whites and cream of tartar for about 2 minutes, or until stiff peaks form. Rinse and dry the beaters thoroughly and set the bowl aside.
Using the hand held electric beater, beat egg yolks for about 2 minutes, until thick. Add sugar in 3 batches and continue to beat for about 4 more minutes, until thick and falls back as ribbons when beater is lifted. Add ½ cup milk and beat for a few seconds, until mixed well.
Switch to a rubber spatula and fold in flour mixture in 2 additions. Then, fold the beaten egg whites into the batter gently, scraping the bottom and sides of the bowl, until combined.
Pour batter into prepared pan, spreading into an even layer. Bake for 20 to 23 minutes, until slightly golden on top and a toothpick inserted in the middle comes clean. Place pan on a wire rack to cool completely.