Whisk lemon juice, zest, sugar, milk, and cream in a medium bowl until sugar dissolves (5 minutes)
Whisk 4 egg yolks in a separate bowl, then gradually add 1/2 cup of the custard mixture while whisking constantly
Return tempered mixture to a saucepan, cook over medium heat until it coats the back of a spoon (80°C/170°F) without boiling
Strain into heatproof bowls, cover with plastic wrap pressed to the surface, and refrigerate 4 hours until fully set
Top with additional lemon zest and whipped cream before serving