In a medium heavy saucepan whisk together the egg yolks and sugar, once combined add in the lemon zest and juice; whisk for one minute.
Place saucepan over medium low heat and whisk constantly until mixture thickens (about 12 minutes).
Once curd has thickened, remove from heat and add in the pieces of butter; let sit for a minute to melt. Then stir the curd and butter together until the curd is room temperature
Transfer the curd to a small bowl or mason jar, cover and store in the fridge until ready to use.
