Whisk together zest, juice, sugar, and egg yolks in a metal bowl and add butter.

Set bowl over a saucepan of simmering water and cook, whisking, until smooth and coats the back of spoon, about 5 minutes.

Spoon curd through a fine sieve set into another bowl, if desired.

Serve warm or store curd in a covered 8 ounce jar in the refrigerator for up to 1 week.