Here is the recipe for lemon curd.
In a medium sauce pan whisk together lemon juice, egg yolks, zest and sugar.
Cook over medium heat and stir constantly until the mix is thick enough to coat a spoon.
Strain into your container.
Place butter into lemon curd and allow to rest and melt below the surface. Stir occasionally to emulsify.
When butter is melted, stir to completely emulsify and cover with a layer of plastic film and the container lid. Refrigerate until ready to use.
Use room temperature butter.
Stir mixture continuously until done.