6 egg yolks, beaten
1 cup sugar
½ cup fresh lemon juice, strained
½ cup unsalted butter (Cold and cut into small pieces)
1 tablespoon lemon rind, grated
Beat the egg yolks until well-blended. Strain the egg yolks through a sieve into a medium-size heavy saucepan. (Sieving removes any shells or albumen; heavy saucepan prevents curdling.) Add the sugar and lemon juice, stir to combine, and cook over low heat, stirring constantly, for about 10 to 15 minutes. Do not hurry the process or else the eggs will curdle.
Cook until the mixture coats the back of a wooden spoon. Remove from heat and immediately pour through another sieve to catch any "scrambled egg bits". Pour into a heat-proof measuring cup and stir in the cold butter pieces, which cools down the mixture quickly. Add the rind, continue stirring until well-blended.