Mix the sugar and the lemon zest until the sugar has the texture of wet sand and the lemon zest is fragrant. You could also pulse them a couple of seconds in a mini chopper.
Add the yolks, whole egg, lemon juice, and salt and mix well until you have a homogenous liquid.
On a double boiler, or in a saucepan over low heat, cook for about 10 minutes, mixing continuously.
Remove from heat and incorporate the butter one tablespoon at a time.
Place in a container and cover with plastic wrap, placing the wrap on the surface of the curd and pushing out as many air bubbles as you can. This will prevent your curd from getting a skin.
This lemon curd keeps well in the fridge in an airtight container for up to two weeks. You can freeze it for up to a year if protected well from freezer burn.