Preheat your oven to 350 degrees.

Combine sugar and flour.

Add lemon zest, juice, and melted butter.

Stir to combine.

In a separate bowl, combine your egg yolks and milk.

Add to flour mixture and stir just until combined.

In another bowl, beat egg whites until stiff peaks begin to form.

Gently fold your eggs whites into the batter using a spatula.

Scoop batter into your 9x13 pan.

In your larger pan, pour enough hot water to cover an inch of the pan. (Hot from the tap is fine, don't boil this water.)

Bake for 40 minutes or until the top is golden brown and springs back when gently pressed.

Best served warm.