8 eggs, separated
2/3 c. sugar
1 lemon zest (whole lemon), grated
1 tbsp. flour
Juice of 4 lemons
1 tbsp. softened butter
Pinch of salt
Pinch of cream of tartar
1 10" tart shell, baked

Preheat oven to 400 degrees. Using an electric mixer, beat egg yolks with 1/3 cup sugar, lemon zest and flour until thick and pale yellow. Transfer to double boiler and add lemon juice. Over medium heat, whisk until thick and set. Remove from heat and whisk in butter. Cover and set aside.