Combine sugar, corn starch and salt in a heavy saucepan; stir together with a wire whisk until thoroughly mixed. Continue to whisk, adding the milk by droplets at first, then in a continuous stream. Whisk in egg yolks, stirring vigorously until blended. Over medium heat, cook the mixture, stirring constantly until it boils. When the mixture reaches a full boil, reduce heat to low and continue cooking and stirring for 3 minutes. Remove mixture from heat; beat in margarine or butter, peanut butter and vanilla. Cover the entire surface of the pudding with plastic wrap and allow to cool for 15 minutes. Stir well and pour into baked pie shell. Best served at room temperature, but can be chilled in refrigerator if desired.