In a small heavy saucepan, whisk together egg yolks, sugar, lemon zest, and lemon juice until blended. Add butter; cook over medium heat, whisking constantly, until mixture is very thick and a thermometer reads at least 175, about 10 minutes. Do not let boil. Immediately remove from heat.
Strain through a fine-mesh sieve into a small bowl. Press plastic wrap onto surface, and let cool for 15 minutes. Refrigerate until serving, at least 1 hour.
Recipe by Victoria at https://victoriamag.com/lemon-lavender-scones-lemon-curd/