Preheat your oven to 325°F (160°C) and line an 8x8-inch baking pan with parchment paper, lightly greasing it.

In a mixing bowl, whisk together the egg yolks and sugar until pale and thick (3-5 minutes). Incorporate melted butter, lemon zest, lemon juice, and vanilla extract.

Gradually whisk in all-purpose flour and salt. Slowly add warm milk while stirring to create a smooth batter.

Beat egg whites in a separate bowl until stiff peaks form. Gently fold one-third into the batter, then incorporate the rest carefully to maintain airiness.

Pour the batter into the prepared baking pan and bake for 40-45 minutes until golden and springy to touch.

Let the cake cool completely in the pan for about an hour, then refrigerate for at least another hour to set the custard layer.

Cut the chilled cake into squares and dust with powdered sugar just before serving.