Whisk the egg yolks together in a mixing bowl and set the bowl next to stove.
In a medium saucepan set over medium-high heat, stir together sugar, cornstarch, and salt, then whisk in the 3 ¼ cups milk. Bring to a simmer.
Whisk about ½ cup hot milk mixture into the bowl with the egg yolks, then pour the egg yolks back into the saucepan. Continue whisking until mixture come to a boil and starts to thicken.
Reduce heat to a gentle boil and whisk for another 2-3 minutes. Remove from heat and stir in chocolate, vanilla, and the 2 teaspoons butter.
Pour chocolate filling into a medium metal bowl, then set bowl in a larger bowl of ice and cold water and let cool 5 minutes, stirring constantly, until filling reaches room temperature. Pour filling into prepared crust and cover with plastic wrap or a piece of well-buttered foil, pressing it directly onto surface to prevent skin from forming. Chill until cold, at least 3 hours and up to 1 day.
Just before serving (or you can do this as soon as the pie is cold), whip the cream and spread freshly whipped, sweetened cream (or whipped topping) over top.
Notes
For the crust, use your favorite recipe. I generally use 1 ⅓ to 1 ½ cups crumbs, ⅓ cup sugar (sometimes less) and about ⅓ cup butter and bake for 8-10 minutes. To help loosen the pie from the pie plate, set the cold pie in a rimmed baking sheet filled with hot water. The heat from the hot water will melt the cold butter somewhat and make it easier to lift it.