Warm the milk and butter together over low heat until the butter melts, then set aside to cool slightly

Beat the egg whites in a clean bowl until stiff peaks form and hold their shape

In another bowl, whisk egg yolks, sugar, lemon zest, vanilla, and salt until pale, thick, and creamy

Add both flours gradually and mix until a smooth, thick batter forms

Slowly pour in the warm milk mixture while mixing on low speed until fully incorporated

Let the batter rest briefly so it slightly thickens and settles

Gently fold in the whipped egg whites, leaving small visible lumps for a light texture

Pour the thin batter into the prepared pan and smooth the surface lightly

Bake until the top turns golden and the center still jiggles slightly when shaken

Cool at room temperature, then refrigerate until fully set for clean slices

Dust with powdered sugar and add extra lemon zest before serving