Bake the pie crust per the package directions until golden brown. Cool.
In a small mixing bowl, whisk together the sugar, cornstarch, salt and egg yolks until fully combined.
On the stovetop in a heavy bottomed saucepan heat together the eggnog and 2 Tbsp butter. Cook on medium-high until the butter has melted, whisking constantly. Whisk the sugar-egg mixture into the eggnog.
Continue to cook whisking constantly over medium heat until thickened. Simmer for 1 minute then remove from the heat. Add the vanilla. Set aside.
Melt together the bittersweet chocolate and 2 Tbsp butter in the microwave in a medium bowl. Heat in 20 second increments stopping to stir each time. Repeat the process until smooth.
Whisk the melted chocolate into the eggnog custard until fully combined. Pour into a large bowl and press a piece of plastic wrap on top.
Chill in the fridge for 1 hour or just until the custard has cooled completely.
After chilling, remove the plastic wrap and give the filling a good stir.
Spread the chocolate eggnog filling into the pie crust. Return to the refrigerator and allow to chill for 4 hours or until completely set.
Serve decorated with whipped cream.
Homemade Whipped Cream: Chill a medium bowl and beaters in the freezer for 15 minutes. Remove from the freezer and whip 2 cups whipping cream with 1 teaspoon vanilla extract. Gradually add 1/4-1/3 cup granulated sugar, whipping on medium-high speed until stiff peaks form. Don't overbeat. Chill until serving. Fresh whipped cream is best made and served the same day.