Whisk together evaporated milk and egg yolks in medium saucepan. Heat over medium-low heat, stirring constantly, until mixture is very hot and thickens slightly; DO NOT BOIL! Remove from heat; stir in morsels until completely melted and mixture is extremely smooth. Pour into crust; refrigerate 3 hours or until firm. If using whipping cream to make topping, prepare, sweetening to taste and chilling. Fill piping bag with topping and pipe on with decorative fitting. If using Cool-whip, apply a thick layer and spread evenly. Finish top with curls of dark chocolate.