Custard Eggnog

Heat milk in a large saucepan over medium heat until small bubbles appear on the side of pan. Whisk together sugar, flour and salt in another large, heavy, nonaluminum saucepan. Add whole eggs and egg whites and whisk until smooth. Gradually blend in the scalded milk. Cook over low heat, stirring constantly, until the custard has thickened enough to coat a spoon, about 15 minutes. Remove from heat and strain into a bowl. Cover; chill at least 2 hours or overnight. Before serving, stir in vanilla, nutmeg and liquor (if using). Pour 2 cups of the custard into a blender, add 2 ice cubes and whirl until frothy. Repeat with remaining custard.