In a heavy saucepan, combine the egg yolks and sugar; whisk briskly for about 1 minute. Add the lemon zest and juice; whisk again for 1 minute to blend. Cut butter into small pieces, about 1 inch each.
Set the pan over low heat and cook gently, stirring almost constantly, until the mixture visibly thickens, about 15 minutes.
Remove pan from heat and add pieces of butter. Let cool, stirring occasionally, until butter is melted and mixture comes approximately to room temperature.
Cover tightly and store in the fridge. Makes 1 cup worth.
Curd will keep well in the fridge for about 7 days.
Cooking low and slow, and the near-constant stirring, is important to prevent the egg from "cooking" into little clumps, rather than forming the sweet custard that you want.