Prepare the Batter

1. Preheat the Oven: Preheat your oven to 325°F (160°C) and grease an 8×8-inch baking dish or line it with parchment paper.
2. Separate the Eggs: Separate the egg whites and yolks, placing them in two large bowls.
3. Whip the Egg Whites: Using an electric mixer, beat the egg whites until stiff peaks form. Set aside.
4. Mix the Yolks and Sugar: In the second bowl, whisk the egg yolks and granulated sugar until light and creamy.
5. Add Butter and Vanilla: Slowly add the melted butter and vanilla extract to the yolk mixture, whisking until smooth.

Combine the Ingredients

6. Incorporate the Flour: Sift the flour and salt into the yolk mixture, stirring until well combined.
7. Add the Milk: Gradually pour in the lukewarm milk, whisking constantly to create a smooth batter.
8. Fold in the Egg Whites: Gently fold the beaten egg whites into the batter in three additions. The batter will be thin and slightly lumpy—this is normal.

Bake the Cake

9. Pour and Bake: Pour the batter into the prepared baking dish and smooth the top with a spatula. Bake for 50-55 minutes, or until the top is golden brown and the center jiggles slightly when shaken.
10. Cool Completely: Allow the cake to cool in the pan for 30 minutes, then refrigerate for at least 3 hours to set the layers.

Serve the Cake

11. Slice and Dust: Slice the cake into squares and dust the top with powdered sugar before serving.