Place a fine-mesh sieve over a small bowl and have it ready by the stove.Advertisement
Bring about 1 inch of water to a simmer in the bottom of a double boiler.
Whisk milk, egg yolk and sugar in the top of the double boiler to combine. Place over the simmering water and cook, stirring constantly and adjusting the heat as necessary to maintain a gentle simmer, until the sauce steams, registers at least 160 degrees F on an instant-read thermometer and thickens enough to coat the back of a spoon, 4 to 8 minutes.
Immediately pour the custard through the sieve into the clean bowl. Whisk in kirsch (or Grand Marnier), if using. Press plastic wrap directly onto the surface of the sauce and refrigerate until chilled, at least 1 1/2 hours.
Make Ahead Tip: Cover and refrigerate for up to 2 days.