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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.
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2
Meanwhile, in a medium bowl, whisk the eggs, egg yolks, and sugar together to blend them, then whisk in the dissolved cornstarch.
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3
Whisking constantly, slowly pour the hot butter and lemon juice into the yolk mixture.
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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.
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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.
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6
Immediately remove from the heat, strain into a small bowl, and stir in the lemon zest.
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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.
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8
The sauce will thicken further as it chills.
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9
The sauce can be refrigerated for up to 3 days.
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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.
