Place lemon zest in the bottom of a medium bowl and set to the side.
Add butter, granulated sugar, and egg yolks to a medium saucepan and whisk to combine. This will be a thick mixture.
Whisk in the lemon juice and salt. It is likely the mixture will look curdled at this point. Don’t worry, it will smooth out while cooking.
Cook the lemon curd over medium-low heat until thickened. Stir the lemon curd constantly with a silicone spatula making sure to scrape the sides and bottom of the pan to avoid burning the mixture. It will take 10-15 minutes to cook the curd, but don’t be tempted to increase the cooking temperature to speed the process up. It’s very easy to overheat the curd. If it reaches boiling, it will cause the eggs to scramble, and the lemon curd will look curdled.
Cook the curd until it reaches nappe (i.e., when the curd coats the back of a spoon and maintains a line when a finger is drawn across the spoon instead of dripping down to fill in the line) or approximately 180°F/83°C. You can also tell it’s ready if you drop some of the curd back into the pot and it leaves a small pool or trail for a few seconds before sinking back into the curd.
Immediately pour the curd through a strainer and into the bowl with lemon zest. Don’t let the curd sit in the hot pot because it will continue to cook and will likely burn.
Stir the lemon zest into the lemon curd to incorporate it and then pour the hot curd on top of the baked shortbread.