1. Preheat your oven to 325°F (160°C). Grease and line an 8×8-inch baking dish with parchment paper.

2. Separate the eggs. In a clean bowl, beat the egg whites to stiff peaks and set aside.

3. In another large bowl, whisk the egg yolks with sugar until pale and creamy. Slowly add the melted butter and mix until smooth.

4. Whisk in the flour, then gradually add the milk while stirring to combine. Add the lemon juice, zest, vanilla extract, and salt.

5. Gently fold the whipped egg whites into the batter. Stop when streaks remain for a marbled, airy batter.

6. Pour the mixture into the prepared baking dish. Bake for 40 to 50 minutes, or until the top is golden and the center has a slight jiggle.

7. Let cool completely in the pan. Refrigerate for 1 to 2 hours for cleaner slicing.

8. Dust with powdered sugar before serving. Garnish with lemon slices or mint if desired.