A couple of egg yolks are all you need to make a thick, rich cream sauce.
Whisk egg yolks, sugar and salt in a small bowl until smooth.
Heat milk and coffee granules in a small heavy saucepan over low heat until steaming. Gradually add to the egg mixture, whisking constantly. Return mixture to saucepan. Cook over low heat, stirring constantly with a wooden spoon, until thick enough to coat the back of a spoon, 7 to 8 minutes (an instant-read thermometer should read 160 degrees F). Do not let sauce simmer.
Strain the sauce through a fine sieve into a clean bowl. Stir in vanilla. Cover with plastic wrap and refrigerate until chilled, about 2 hours.