Pastry Cream (creme Patisserie)

Mix egg yolks and sugar in a 2 1/2 quart saucepan with a wire whip until thick and pale yellow, several minutes. It should form a ribbon. Beat in flour, then beat in hot milk in a thin stream. Set over moderate heat and stir slowly and continuously with the whip until it thickens. When lumps form, beat vigorously to eliminate them. Lower the heat and cook slowly, stirring all the time, to thicken the cream. Remove from heat and beat in the butter and flavoring. Film the top of the cream with 1/2 tablespoon of milk, rum, or kirsch to prevent crusting. Chill. You need a good saucepan for this - one that will heat evenly. Be thorough in your stirring so none of the cream sticks to the bottom of the pan and turns nasty. Having said that, this recipe is actually pretty easy if you are patient and calm about it, and the results are very, very good.