Step 1: Preheat the Oven and Prepare the Baking Dish

Start by preheating your oven to 325°F (165°C). This will ensure the oven reaches the right temperature when you’re ready to bake. While the oven is heating up, lightly grease an 8×8-inch baking dish with butter or nonstick cooking spray. This step is crucial to make sure the cake comes out easily once it’s finished baking.

Step 2: Whip the Egg Whites

In a separate bowl, use a hand mixer or stand mixer to beat the egg whites until stiff peaks form. This is a key step, so be sure to get the egg whites nice and stiff. They should hold their shape when you lift the whisk from the bowl. You can test by lifting the whisk out—the egg whites should form firm peaks that stand up straight. Don’t overbeat the egg whites, though, as they can become dry and difficult to fold in later.

Step 3: Mix the Egg Yolks and Sugar

In another large bowl, beat the egg yolks and powdered sugar together until the mixture turns pale yellow and smooth. This will take about 2-3 minutes with an electric mixer. The sugar will dissolve into the yolks, creating a creamy base for your cake batter.

Step 4: Add Butter, Water, and Flour

Once the yolk-sugar mixture is smooth, add the melted butter and water. Beat this in for about 2 minutes, making sure it’s well combined. Then, sift in the flour, mixing until fully incorporated. This will create a smooth batter without any lumps, so be sure to mix until the flour is fully absorbed.

Step 5: Mix in Milk and Vanilla Extract

Now it’s time to add the milk and vanilla extract. Gradually pour the lukewarm milk into the batter while mixing. Continue mixing until everything is smooth and well combined. Add the vanilla extract and stir to incorporate. The batter should be fairly thin at this point, but that’s completely normal!

Step 6: Fold in the Egg Whites

Take about a third of the whipped egg whites and gently fold them into the batter. Use a spatula to carefully lift and fold the mixture, ensuring you don’t deflate the egg whites. Once this third is fully incorporated, fold in the remaining egg whites in two more additions. Be as gentle as possible during this step so you retain the air in the egg whites, which will help create the fluffy top layer of the cake.

Step 7: Pour the Batter Into the Pan

Once the egg whites are fully incorporated, pour the batter into the prepared baking dish. The batter will seem quite thin, but that’s the magic of this cake—it’s supposed to be this way.

Step 8: Bake the Cake

Bake the cake for 40-60 minutes at 325°F (165°C). Check the cake after 40 minutes. The top should be golden and set, with only a slight jiggle in the center. If the cake begins to brown too quickly before the minimum baking time, you can cover the top with aluminum foil to prevent it from burning. Every oven is different, so the exact baking time may vary, but start checking after 40 minutes.

Step 9: Cool the Cake

Once the cake is finished baking, let it cool completely before dusting with powdered sugar. The custard filling will still be slightly jiggly, which is totally fine. The top should be golden and lightly puffed, and the middle will have a creamy custard-like consistency. This is part of what makes the cake so magical!