Magic Cake (Gâteau Magique)

Preheat the oven to 325°F. Grease an 8-inch square pan with butter or cooking spray. Line the pan with a layer of parchment paper covering the bottom and up 2 sides with a slight overhang. Lightly grease the parchment paper.

In a small saucepan, heat the milk and butter on low heat just until the butter melts. Don't let the milk come to a boil or simmer. It should be warm but not scalding hot, 105 to 115°F. Set aside.

In a large bowl using a hand mixer, beat the egg whites on medium-high speed until they form stiff peaks, about 4 minutes. When you lift the beaters out of the bowl, the whipped egg whites should stand up at a point and hold that point.

In another large bowl, beat the egg yolks, sugar, vanilla extract, salt, and nutmeg (if using) on high speed until the mixture is pale and creamy, about 2 minutes. Add the flour to the egg yolk mixture and beat on low speed until combined. It may seem like there is too much flour, but the batter will soon be thinned out.

Continue mixing on low speed, or switch to a whisk, and pour in the warm milk and butter a splash at a time. Once you've incorporated about half of the milk mixture, you can pour in the rest, and whisk to combine. The batter will be thin and smooth, like crepe batter. Add the beaten egg whites and use the whisk to gently mix until there are no large lumps of egg whites. Don't overmix—there should still be a layer of foam on top of the batter.

Pour the batter into the prepared pan. Bake until the top is golden brown, the cake has begun to pull away from the sides of the pan, and the center has a slight jiggle when the pan is gently shaken, about 50 minutes.

Remove the pan from the oven and set it on a wire rack until it's cool enough to go in the fridge, about 30 minutes. Let the cake chill in the fridge for at least 1 hour before serving. You can skip chilling and serve the cake at room temperature, though it won't slice as neatly. Dust the top of the cake with powdered sugar, then slice it into squares. If desired, serve with fresh fruit and, for extra indulgence, a dollop of whipped cream. Store leftover magic cake, well-covered, in the refrigerator for up to 3 days. Magic cake does not freeze well. The freezing and thawing process will affect the texture of the custard layer.