Preheat oven to 375 degrees F. Rub a 9 or 9 ½ inch deep dish pie plate with butter.

In a bowl, stir together the crumbs and butter. Press into bottom and up sides of pie dish and bake for 5 minutes. Let cool.

Whisk the egg yolks 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 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. Reduce heat slightly. Whisk and stir until mixture thickens, then continue whisking 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 until stiff peaks start to form. Add sugar, then add Irish Cream 1 tablespoon at a time until it is blended in (do not overbeat the cream). Pipe or spread over top of pie.