To make lemon curd, strain the beaten egg yolks through a sieve into a medium-size heavy saucepan. Add the sugar and lemon juice, stir to combine, and cook over low heat, stirring constantly, for about 10 to 12 minutes, until the mixture thickens and coats the back of a spoon.
Remove from the heat and stir until the mixture cools just slightly. Stir in the butter, a piece at a time, until fully incorporated. Add the rind. Let cool completely.