In a medium bowl, combine egg yolks, 3 tbsp sugar, and cornstarch; set aside.
Bring milk, remaining sugar, and salt to simmer in a medium saucepan; stir frequently.
Temper the egg mixture by removing 1 cup of milk from the saucepan and slowly pour it into the egg mixture while whisking vigorously.
Slowly pour egg mixture into saucepan while whisking. Continue to whisk while bringing the temperature to medium heat. The milk and egg mixture will begin to thicken. Remove from heat when small bubbles begin to pop on the surface (about 30-60 seconds).
Whisk in butter and chocolate until smooth. Stir in vanilla.
Pour into prepared chocolate nut pie crust. Cover with plastic wrap. Place in the refrigerator for a minimum of 4-6 hours.
Prepare whipped topping in a cold stand mixer bowl. Whisk milk, sugar, and vanilla on medium speed until the cream thickens.
Spread whipped cream over the chocolate pie and decorate with chocolate shavings.