Bake the pie crust per the package directions until golden. 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 until thickened. Simmer for 1 minute then remove from the heat. Add the vanilla. Set aside.
Melt together the bittersweet chocolate chips and 2 Tbsp butter in the microwave. Heat in 20 second increments stopping to stir each time. Repeat until smooth.
Whisk the melted chocolate into the custard until fully combined. Pour into a bowl pressing plastic wrap on top.
Chill for 1 hour or until the custard has cooled completely.
After chilling, remove the plastic wrap and stir.
Spread into the pie crust. Chill for 4 hours or until completely set.
Serve with a dollop of whipped cream.