3/4 cup cold water, divided
1 teaspoon unflavored gelatin
1 cup sugar
3 tablespoons cornstarch, sifted
8 egg yolks, lightly beaten
1 cup fresh lemon juice
4 tablespoons unsalted butter
2 tablespoons fresh lemon zest
In a small bowl, combine 1/4 cup cold water and gelatin; let stand 5 minutes. In a separate small bowl, combine sugar and cornstarch.
Meanwhile, in a medium saucepan, whisk together sugar mixture, remaining 1/2 cup cold water, egg yolks, and lemon juice until combined. Cook over medium heat, stirring constantly, until mixture begins to boil, about 7 minutes.
Remove from heat; add gelatin mixture, whisking to combine. Add butter, 1 tablespoon at a time, stirring until melted. Strain mixture. Stir in zest.
Place a piece of plastic wrap on the surface of the curd. Refrigerate at least 4 hours. Store in an airtight container in refrigerator up to 5 days.