In a bowl set over a pan of simmering water, whisk together lemon juice, butter, sugar, eggs and salt until butter melts and mixture thickens and coats back of a wooden spoon, abut 25 minute (do not let it boil).
Strain mixture through a fine-mesh sieve into another bowl and stir in zest. Cover with plastic wrap, pressing directly onto surface. Let cool to room temperature, then refrigerate until chilled.
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