In the top of a double boiler set over—but not touching—a pot of barely simmering water, combine the butter, brown sugar, and whisky. Whisk until the butter and sugar are melted and the mixture is smooth.
Add the yolks and continue whisking (or stirring with a wooden spoon) until the sauce is very thick and has some body, 7 to 10 minutes; it will have the consistency of caramel sauce (an instant-read thermometer should register 160°F). If you want more whisky flavor, add more, 1 tsp. at a time, to taste. Serve hot or warm. This sauce thickens as it cools.
nutrition information (per serving):
Photo: Scott Phillips