In a double boiler or metal bowl over simmering water, heat the milk, butter and chocolate until chocolate is melted (chocolate may appear curdled)., Combine sugar, flour and salt. Sprinkle over chocolate mixture (do not stir). Cover and continue to cook in a double boiler over medium-low heat for 20 minutes. With a wooden spoon, stir until smooth. Remove from the heat., Stir a small amount of hot mixture into egg yolks; return all to the pan, stirring constantly. Cook and stir until mixture is thickened and a thermometer reads 160°. Remove from heat; stir in vanilla. Cool for 15 minutes, stirring occasionally. Transfer to dessert dishes., Cover and refrigerate for 1 hour. If desired, top with whipped cream.