PREPARATION: Grate the zest of 6 lemons and set aside in a bowl with the sugar. OR, use a vegetable peeler to remove 3/4 cup zest from the lemons and process the lemon strips with the sugar in a food processor until zest is minced, about 30 seconds. Set lemons aside. COOKING: Whisk the egg yolks with the cream and salt in a heatproof mixing bowl. Put the half-and-half and lemon-sugar in a medium nonaluminum saucepan. Bring liquid to a simmer, stirring to dissolve the sugar. Remove pan from heat. Slowly whisk the hot half-and-half into the egg yolk mixture. Then return the partially cooked custard to the saucepan. Stir constantly over medium-low heat until mixture begins to thicken slightly and coats the back of a spoon, about 5 minutes (the approximate consistency of unwhipped heavy cream). Pour hot custard into a bowl without straining. Stir in the vanilla. Put a sheet of plastic wrap directly on the surface of the custard to prevent skin from forming. Set custard aside at room temperature to cool. (Can cover with plastic wrap and refrigerate for up to 48 hours.) FREEZING: Squeeze 3/4 cup juice from the lemons and stir juice into cooled custard. Transfer the custard mixture to an ice cream maker and freeze according to manufacrurer's instructions. Makes about 1 1/2 quarts. [COOKS; Jul/Aug 1988]