In a large mixer, whisk together softened butter, sugar, and lemon zest until a sandy texture is reached.
Add eggs one a time, not adding another egg until each is fully incorporated.
Pour in lemon juice and continue to mix for 30 seconds.
Using a nonstick pan, pour in lemon mixture and heat over low heat until thickened. NOTE: The temperature of the mixture should not exceed 170° and stirred constantly to prevent curdling of egg whites. If curdling happens, strain mixture through a fine mesh strainer.
Lemon curd can be kept in the refrigerator (in an airtight container) for up to 10 days.