Mix sugar, flour and salt in a heavy 6-quart saucepan. Add egg yolks and mix well. Using a wire whisk, stir the egg mixture and slowly pout in the milk and cream. Cook over low heat, stirring constantly, until custard is thickened and just coats the back of a wooden spoon. Remove from the heat and let cool. The custard will thicken as it cools. When it is cold, stir in vanilla, nutmeg and whiskey. Serves 12.