What You Need
4 large egg yolks
4 large egg whites, at room temperature
3/4 cup granulated sugar
2 tablespoons cold water
8 tablespoons unsalted butter, melted
3/4 cup all-purpose flour
1/8 teaspoon fine salt
2 cups whole milk, half-and-half, or light cream
2 teaspoons vanilla extract or vanilla bean paste
8-inch square pan (2 inches deep)
2 large bowls
Handheld electric mixer
Prepare the pan: Coat an 8-inch square pan that is 2 inches deep with cooking spray. Cut 2 (8- by 12- to 16-inch) sheets of parchment paper. Lay one sheet in the pan, leaving an overhang of at least 2 inches on each side. Lay the second piece on top, across the first, leaving an overhang on the other sides. These will be your “handles” when you take the cake out of the pan; set aside.
Heat the oven: Arrange a rack in the middle of the oven and heat to 325°F.
Mix the yolk, sugar, and water until pale and fluffy: In a stand mixer fitted with a whisk attachment, or in a large bowl using a handheld electric mixer or a whisk (and plenty of arm power), mix the egg yolks, sugar, and water and mix until the sugar is dissolved, very fluffy, light in texture, and the mixture is a very pale yellow, 5 to 6 minutes. Do not undermix.
Drizzle in the melted butter: While continuing to mix, drizzle the melted butter into the egg yolk mixture, and mix until it is well-incorporated. Scrape down the bottom and sides of the bowl.
Stir in the flour and salt: Add in the flour and salt and mix until fully combined, scraping down the bottom and side of the bowl as needed. Add the milk or cream and vanilla and mix them in gently and on a slow to medium speed until fully incorporated.
Whisk the egg whites: If you are using a handheld mixer or whisk, pour the egg whites into a large, clean, dry mixing bowl. If you are using a stand mixer, wash and dry the bowl and whisk attachment and pour the whites into it. Whisk on high speed until the egg whites hold stiff peaks, about 2 minutes.
Add the egg whites to the yolk mixture: Add the whipped egg whites to the egg yolk and flour mixture and mix gently with a large spoon or a whisk only, about a dozen stirs, making sure to leave a distinct layer of fluffy lumps and clumps remaining at the top. Do not overstir or overmix.
Bake: Pour the mixture into the prepared pan and bake until the edges are set, the top is a warm to deep brown, and the center still wobbles and wiggles when the pan is jiggled, 50 to 55 minutes.
Cool, then chill the cake: Let the cake sit in the pan at room temperature until completely cooled. Loosely cover it with plastic wrap and refrigerate for at least 4 hours and up to 12 hours before serving.
Storage: This cake can be stored in an airtight container in the refrigerator for up to 2 days.