Prep the pan and oven: Preheat the oven to 325°F (160°C). Line an 8-inch (20 cm) square baking pan with parchment, leaving overhang for easy lifting. Lightly grease the sides.

Warm the milk: Heat the milk gently until just warm to the touch (not hot). This helps the batter come together and promotes clean layering.

Separate the eggs: Place egg whites in a clean bowl and yolks in a large mixing bowl. Make sure no yolk gets into the whites.

Whip the egg whites: Add a pinch of salt to the whites and beat until stiff peaks form. Set aside. The whites should be glossy and hold their shape.

Whisk the yolks and sugar: In the bowl with yolks, whisk with the granulated sugar until pale and slightly thick, about 2 minutes.

Add butter and vanilla: Whisk in the melted, cooled butter and the vanilla. If you’re using lemon zest, add it now.

Incorporate the flour and salt: Sift the flour and the remaining salt over the yolk mixture. Whisk until smooth and no dry pockets remain.

Stream in the milk: Gradually whisk in the warm milk in 2–3 additions. The batter will look thin—this is right.

Fold in the egg whites: Add the beaten whites in three parts, gently folding. Leave some small, visible puffs of white throughout the batter. Do not overmix; those airy pockets help form the layers.

Pour and bake: Pour the batter into the prepared pan. Bake for 45–55 minutes, until the top is golden and set but still slightly jiggly in the center when you nudge the pan. A light crackle on top is normal.

Cool completely: Let the cake cool in the pan on a rack for at least 1 hour. Then refrigerate for 2–3 hours to firm the custard layer.

Slice and serve: Lift out using the parchment. Dust with confectioners’ sugar. Cut into squares with a sharp knife, wiping the blade between cuts for clean edges. Serve chilled or at cool room temperature.