Place a medium glass bowl in the refrigerator to chill.
In a heavy nonreactive saucepan combine the egg yolks, sugar, flour, and salt. Beat until the mixture is pale yellow. Place over low heat and slowly stir in the warm milk. Cook, stirring constantly, until custard coats a wooden spoon quite thickly, about 5 minutes.
Immediately pour the custard into the chilled mixing bowl; cover and place in the refrigerator until custard is cold, about 2 hours. When the custard is chilled, stir in the bourbon.
Serve in dessert cups, topped with sweetened whipped cream. Sprinkle a little nutmeg or cinnamon over the whipped cream.