In saucepan, whisk together egg yokes, sugar, lemon juice and salt.
Place pan on medium-low heat on stove top. Once it begins to warm, add butter and lemon zest.
Stir constantly while the curd heats. DO NOT LET IT BOIL. Cook it until it is thick enough to coast the back of a spoon. Temperature will be in the 160-degree range. Place your thermometer in the pan at the beginning and keep an eye on it.
Remove from heat and strain through a wire mesh strainer. Make sure you're straining into a bowl that can tolerate the heat.
Cover the surface with plastic wrap, pushing the wrap to seal the top, so a film doesn't form. Allow to cool completely before placing it in the refrigerator.
This curd will makek enough for four-plus macarons.