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