Finely grate enough zest from lemons to measure 2 teaspoons and squeeze enough juice to measure ½ cup.
Whisk together zest, juice, sugar, and eggs in a metal bowl and add butter.
Set bowl over a saucepan of simmering water and cook, whisking constantly, until thickened and smooth and an instant read thermometer registers 160°F., about 5 or 6 minutes.
Force curd through a fine sieve set into another bowl. Serve warm or cover surface of curd with wax paper and cool completely.
Notes
This can be used as a cake or tart filling or served with scones, muffins or fresh fruit.
Lemon curd keeps, covered and chilled, for about a week.
If substituting regular lemons, increase sugar to ¾ cup.