Lemon Curd

Directions

1. 1
   Whisk egg, egg whites, sugar and lemon juice in a heavy nonreactive saucepan.
2. 2
   Add butter and cook over low heat, whisking constantly, until slightly thickened, 7 to 10 minutes.
3. 3
   Do not simmer.
4. 4
   Pour sauce through a fine-mesh sieve into a bowl.
5. 5
   Stir in lemon zest.
6. 6
   Cover surface with plastic wrap and refrigerate until chilled, about 2 hours.