In a medium bowl, whisk together ¼ cup (50 grams) sugar, zest, and egg yolks. In a medium saucepan, bring lemon juice and remaining ¼ cup (50 grams) sugar to a boil over medium heat. Add about ¼ cup hot lemon mixture to egg yolk mixture, whisking constantly. Add egg yolk mixture to remaining hot lemon mixture, whisking to combine. Strain through a fine-mesh sieve into a bowl, discarding solids.

Return mixture to pan, and cook, stirring constantly, until thickened, 3 to 4 minutes. Whisk in butter, 1 tablespoon (14 grams) at a time, until incorporated. Let cool completely in refrigerator. Cover and refrigerate for up to 1 week.