The Lightest Cake in the World! It Melts in Your Mouth!

Ingredients (for an 8-inch round cake)
5 large eggs, separated
½ cup (100 g) of sugar
1 teaspoon of vanilla extract
½ cup (60 g) of all-purpose flour (sifted)
¼ cup (60 ml) of milk
2 tablespoons (30 g) of unsalted butter
A pinch of salt
Instructions
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