Preheat your oven to 325°F (160°C) and prepare an 8×8-inch baking dish with parchment paper and grease the sides.

Whisk together egg yolks and granulated sugar until pale and thick, about 2-3 minutes. Add melted butter, lemon zest, lemon juice, and vanilla extract, mixing well.

Sift in all-purpose flour and salt, stirring until just combined. Slowly whisk in warm milk until smooth.

In a separate bowl, beat egg whites until stiff peaks form, about 2-3 minutes.

Fold a third of the whipped egg whites into the lemon batter. Add remaining egg whites in two batches, folding gently.

Pour batter into prepared baking dish and bake for 40-45 minutes, until golden and springy.

Cool in the pan for about an hour, then refrigerate for at least 1 hour.

Cut into squares and dust with powdered sugar to serve.