Whisk sugar and egg yolks in a medium bowl until light colored about 3 minutes.
In a medium saucepan heat coffee, cream ,sugar and chocolate liqueur ( if using ) until.bubbles just start to form on edges of pan.
Whisking at all times add a small stream of the cream mixture into the egg yolks until combined
Return liquid to saucepan, and cook, stirring with a rubber spatula continuously reaching all corners of pan until sauce thickens enough to coat the back of a spoon and reaches 170 on a instant read thermometer, about 10 to 15 minutes
Strain into a clean dish, stir in vanilla. Cool uncovered in refrigerator until cold then cover. Serve cold.. This sauce keeps about 5 days in refrigerator. Use on cakes and brownies, it goes with chocolate extremely well. Drizzle on tarts and pies. Also wonderful on coffee cakes!