Preheat oven to 325°F. Line a 9x13 baking pan with parchment and spray sides with nonstick spray.

Separate the eggs, placing whites in one bowl and yolks in another.

Add cream of tartar to the whites and whip to stiff peaks. Set aside.

In the other bowl, whip together the egg yolks, sugar and vanilla for 1 minute until light yellow.

Stir in the milk, melted butter, lemon juice and zest until fully incorporated.

Fold in the flour until just combined.

Gently fold in the whipped egg whites until no streaks remain.

Pour batter into the prepared pan and bake for 40-45 minutes until center is just slightly jiggly.

Allow to cool completely at room temperature, at least 30 minutes.

Dust the top with powdered sugar before slicing and serving.