Chocolate cream pie

This is a pie recipe from Mary Rose and it is so yummy!

3 egg yolks, beaten
1 1/2 cups white sugar
3 tablespoons cornstarch
1/2 cup unsweetened cocoa powder
1/2 teaspoon salt
3 cups milk
1 tablespoon butter
1 1/2 teaspoons vanilla extract
1 cup frozen whipped topping, thawed

Directions
In a large mixing bowl, cream together egg yolks and sugar. Mix in cornstarch, cocoa powder, and salt. Add milk and stir gently.
Pour mixture into a large saucepan and cook over medium heat, stirring constantly, until boiling. Remove from heat. Stir in butter or margarine and vanilla extract. Cool slightly, then pour mixture into pastry shell. Chill before serving. Garnish with whipped topping. Yummy in graham cracker crust too!