For a milder sauce, replace up to half the whiskey with water; for a very potent sauce, replace the water with whiskey.
Melt in a small heavy saucepan over low heat:
Stir in:
Cook, stirring, until the sugar is dissolved. Remove from the heat. Whisk in a medium bowl until frothy:
Slowly whisk the butter sauce into the egg to warm it, then return this mixture to the saucepan. Set over medium heat and, stirring gently, bring to a simmer. Cook until thickened, about 1 minute. Serve at once or set aside at room temperature for up to 1 hour. Let cool, cover, and refrigerate for up to 3 days. Reheat over low heat, stirring; if the sauce separates, remove it from the heat and whisk in a little warm water.