In a medium saucepan, off heat, whisk together sugar, lemon zest, and egg yolks; whisk in lemon juice and salt.
Add butter and place pan over medium-high. Cook, whisking constantly, until butter has melted, mixture is thick enough to coat the back of a spoon, and small bubbles form around the edge of the pan, about 5 minutes (do not boil).
Remove pan from heat while continuing to whisk. Pour curd through a fine-mesh sieve into a glass bowl. Press plastic wrap against surface of curd and refrigerate until cool. To store, refrigerate in an airtight container up to 2 weeks.