Lightly grease a 20 - 30cm baking dish, set aside (square dish)
Whip the egg whites until stiff peaks form, set aside.
Beat the egg yolks and powdered sugar until pale yellow
Add in melted butter and the tablespoon of water, for about 2 minutes, until evenly combined.
Mix in the flour until evenly incorporated.
Slowly beat in the milk and vanilla extract until well combined
Fold in the egg whites, a 1/3 at a time, then repeat until all of the egg whites are folded in.
Pour the batter into the pan and bake for 40-60 minutes, until the cake is barely wobbly in the center.
Baking time might vary depending on the oven or pan you use, but start checking after 40 minutes. If the top browns too quickly before the minimum of 40 minutes, you can cover the cake with aluminum foil.
Cool the cake completely before dusting with powdered sugar. Even cooled, it will be slightly wobbly because it has a custard layer in the center.