In a medium saucepan, blend butter, sugars, salt, and cornstarch. Add evaporaed milk and stir until boiling. Once it boils, cook two minutes longer or until it is thickened. Remove from heat. Blend in pumpkin, spices, and milk. Add some mixture to beaten egg yolks. Add to saucepan and continue to cook for several minutes until it boils. Stir 2 minutes longer. Pour into pie shell. After cooled, add whipped topping.