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 using a mixer and until the sugar is dissolved, very fluffy, light in texture, and the mixture is a very pale whitish-yellow, 5 to 6 minutes. Do not undermix.
Drizzle in the melted butter into the egg yolk mixture, while still mixing and mix until it is well-incorporated.
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.
Using a mixer, beat egg whites on high speed until the egg whites hold stiff peaks, about 2 minutes.
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.
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 wiggles when the pan is jiggled, 50 to 55 minutes.
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.
Add powered sugar or fruit