Make lemon curd:

• In a bowl, rub lemon zest into granulated sugar to release the flavorful oils. Whisk in cornstarch and salt until evenly incorporated.
• Transfer to a medium, heavy-bottomed saucepan set over low heat. Whisk in egg and egg yolks until it forms a thick paste, then whisk in lemon juice until smooth.
• Continue to whisk regularly over low heat until the mixture starts to thicken. You can switch to a spatula at any point if you like. Continue to cook until curd reaches 170ºF and thickens enough that you can swipe your spatula through it and it will not fill in for at least 3-5 seconds.
• Remove from heat and whisk in butter, one piece at a time, whisking until fully melted and incorporated before adding the next.
• Strain through a fine mesh sieve to remove the zest and any bits of cooked egg (this will give you a beautifully smooth texture).
• Let cool completely, or, if not using right away, press a layer of plastic wrap onto the surface of the curd and refrigerate overnight or for up to 5 days of freeze for up to a month.