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

Whisk together the egg yolks until pale and creamy, then stir in the granulated sugar, melted butter, lemon zest, lemon juice, and vanilla extract until smooth.

Sift in the all-purpose flour and salt, whisking to combine, then slowly add the warm milk, stirring to achieve a smooth batter.

In a separate bowl, beat the egg whites until stiff peaks form, then gently fold one-third into the batter followed by the remaining egg whites.

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

Allow the cake to cool completely in the pan at room temperature, then refrigerate for at least 1 hour before slicing.

Dust each piece with powdered sugar before serving.