In a small, heavy-bottomed pot off the stovetop, combine the sugar, cornstarch, Satsuma zest and juice, salt and egg yolks. Whisk together very well until the cornstarch is fully dissolved in the mixture and no lumps remain. Turn the heat to medium and cook, whisking continuously, until the mixture reaches a simmer and begins to thicken, 4 to 5 minutes. Cook another 1 minute, still whisking, then turn off the heat and whisk in the butter until it melts.
Strain the mixture through a fine-mesh sieve, then cover and refrigerate until cooled and set, at least an hour. Refrigerate in an airtight container up to 2 weeks or freeze up to 2 months. Spoon the curd over yogurt or oatmeal, or spread over toast or pound cake.
If you can't find Satsuma mandarins, substitute any variety of orange.