Preheat the oven to 325°F (160°C). Grease an 8-inch square pan with butter or nonstick spray. Line the bottom and two sides with parchment paper, allowing a slight overhang for easy removal. Lightly grease the parchment as well.

In a small saucepan, heat the milk and butter over low heat until the butter melts completely. The mixture should be warm but not boiling (approximately 105–115°F). Remove from heat and set aside to cool slightly.

In a clean bowl, using an electric mixer, beat the egg whites on medium-high speed until stiff peaks form, about 4 minutes. When lifted, the peaks should stand firm. Set aside.

In a separate bowl, beat the egg yolks, sugar, vanilla extract, salt, and nutmeg (if using) on high speed until the mixture turns pale and creamy, about 2 minutes.

Reduce the mixer speed to low and gradually add the flour, mixing until incorporated. The batter will appear thick; this is expected.

While mixing slowly (or whisking by hand), add the warm milk and butter mixture gradually. Begin with small additions, then pour in the remaining milk once it starts blending smoothly. The resulting batter should be thin and fluid, resembling crepe batter.

Add the beaten egg whites to the batter in three portions. Fold gently after each addition using a whisk or spatula. The mixture should remain slightly uneven, with small foamy bits visible—this helps create the cake’s layered texture.

Pour the batter into the prepared pan and smooth the surface gently. Bake for 50 minutes, or until the top is golden brown and the center jiggles slightly when moved. Do not overbake.

Allow the cake to cool on a wire rack for 30 minutes, then refrigerate for at least 1 hour before serving. Dust with powdered sugar before slicing into squares.

Serve chilled or at room temperature with fresh fruit, whipped cream, or a drizzle of caramel sauce for an elevated presentation.