Mix the sugar and lemon zest

• Combine the sugar and lemon zest in a mixing bowl.
• Rub them together so the zest releases its aroma into the sugar.

Whisk in the eggs and butter

• Add the eggs and egg whites and whisk until pale and smooth.
• Pour in the melted butter and whisk again until fully combined.

Add the lemon juice and vanilla

• Stir in the lemon juice and vanilla extract.
• Whisk until the batter is homogeneous.

Fold in the flour

• Add the flour and fold gently until just combined, without overmixing.

Bake

• Pour the batter into a greased or lined baking dish and spread evenly.
• Bake at 180 °C until the top is set and a toothpick comes out with a few moist crumbs.

Cool and cut

• Let the lemon brownies cool completely in the dish.
• Cut into squares and serve.