Serve this tart lemon sauce with Persimmon Pudding from "The Martha Stewart Living Christmas Cookbook."
In a heatproof bowl set over (but not touching) simmering water, mix together sugar, cornstarch, and salt. Add hot water and cook, 3 to 5 minutes, whisking gently, until thick.
Add the butter, lemon juice, and zest and continue cooking, stirring gently, for about 5 minutes. Cool slightly before using, but do not refrigerate (the sauce will become aspic-like) and do not reheat.