Lemon Filling

Directions

1. 1
   In a medium saucepan, heat the butter and lemon juice over medium heat until the butter melts and the mixture is hot, about 130F on a thermometer.
2. 2
   Meanwhile, in a medium bowl, whisk the eggs, egg yolks, and sugar together to blend them, then whisk in the dissolved cornstarch.
3. 3
   Whisking constantly, slowly pour the hot butter and lemon juice into the yolk mixture.
4. 4
   Return the mixture to the saucepan and cook over medium heat, stirring constantly with a large spoon, just until it comes to a boil and thickens; it will take about 6 minutes.
5. 5
   When thickened, the sauce will leave a path on the back of the spoon if you draw your finger across it and look clear rather than cloudy.
6. 6
   Immediately remove from the heat, strain into a small bowl, and stir in the lemon zest.
7. 7
   Press plastic wrap directly onto the surface of the sauce, use a toothpick to poke a few holes in the plastic wrap to let steam escape, and refrigerate until cold.
8. 8
   The sauce will thicken further as it chills.
9. 9
   The sauce can be refrigerated for up to 3 days.
10. 10
    Or, spoon the cold sauce into a plastic freezer container, leaving at least an inch of space in the top of the container, press plastic wrap onto the surface, seal tightly, and freeze for up to 3 months.