In a medium, nonreactive saucepan, whisk together the sugar and cornstarch. Stir in the lemon juice and yolks and heat over medium-low heat. Bring to a boil and allow to boil for 1 minute, stirring continuously, until thickened.
Off the heat and strain through a fine mesh sieve into a glass cup or bowl. To the strained curd, stir in the butter until completely melted. Add the zest and stir to combine.
Gently press plastic wrap directly on top of the curd’s surface and refrigerate for at least 1 hour before using.