Sour Lemon Sauce
- 2 cups sugar
- 2 tablespoons plus 1 1/2 teaspoons cornstarch
- Large pinch of salt
- 2 1/2 cups hot water
- 7 tablespoons unsalted butter
- 1/2 cup freshly squeezed lemon juice
- 2 tablespoons freshly grated lemon zest
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.