Use this aromatic citrus sauce to top ice cream, or try schmearing it on toast, muffins, or scones.
In a medium saucepan, whisk together sugar, both zests, and egg yolks; whisk in citrus juices and salt. Add butter; place pan over medium-high and cook, whisking constantly, until butter has melted, mixture is thick enough to coat the back of a spoon, and bubbles form around edge of pan, about 11 minutes (do not let boil).
Remove pan from heat immediately. Pour curd through a fine-mesh sieve into a glass bowl. Press plastic wrap against surface of curd; refrigerate until cool. Transfer to 6 seven-ounce jars; curd can be refrigerated in jars up to 2 weeks.