In a large mixing bowl use a rotary beater or wire whisk to lightly beat eggs just until mixed. Stir in milk, sugar, 1-1/2 teaspoons vanilla, and the salt.
Place an ungreased 4-1/2- or 5-cup ovenproof ring mold (about 8 inches in diameter) in a 13x9x2-inch baking pan. (Or, place eight 6-ounce custard cups or individual molds in a large roasting pan.) Set the baking or roasting pan on the oven rack. Pour the egg mixture into the mold(s). Pour boiling or very hot tap water into the pan around the mold to a depth of 1 inch. Bake in a 325 degree F oven for 35 to 45 minutes or until a knife inserted near the center comes out clean.
Remove mold(s) from water in pan. Cool custard in the mold(s) on a wire rack. Cover and chill in the refrigerator for 2 to 24 hours before serving.
In a saucepan combine water, corn syrup, cornstarch, and instant coffee crystals. Cook and stir over medium heat until mixture is bubbly. Cook and stir for 2 minutes more. Remove mixture from heat.
Stir in the 1/4 teaspoon vanilla and, if desired, coffee liqueur. Cover and chill sauce in the refrigerator until serving time.
To serve, remove custard from mold(s). Serve sauce with chilled custard. Serves 8.