Preheat the oven to 325°F/160°C. Lightly butter or grease a 8"x8" (20x20cm) baking dish.
Melt the ½ cup (113g) butter and set aside to slightly cool slightly.
Warm the 2 cups (480ml) milk to lukewarm and set aside.
Whip the 4 egg whites and 4 drops vinegar to stiff peaks. Set aside.
Beat the 4 egg yolks and 1 ¼ cups (150g) powdered sugar until light.
Mix in the melted butter and the 1 Tablespoon (15ml) water for about 2 minutes or until evenly incorporated.
Mix in the 1 cup (125g) flour until evenly incorporated.
Slowly beat in the warmed milk and 1 teaspoon (5ml) vanilla extract until everything is well mixed.
Fold in the egg whites, ⅓ at a time. Repeat until all of the egg whites are folded in.
Pour the batter into the prepared pan.
Bake at 325°F/160°C for 45-60 minutes or until the cake is barely jiggly in the center.
If the top browns too quick before the minimum of 45 minutes, then cover the cake with aluminum foil and cook for remainder of time needed.
Allow cake to completely cool before cutting and then dust with powdered sugar.
For faster cooling you can place the cake in the fridge.
