Make pudding: Melt butter in a pan over medium heat, stir in dark brown sugar and salt and cook, stirring constantly, 3 minutes. Remove from heat. In a small bowl, whisk cornstarch and 1/2 cup milk until smooth, then whisk in egg yolks.
Whisk remaining milk into brown sugar mixture, then whisk in cornstarch mixture. Return pan to medium heat and bring to a boil, whisking often. Reduce heat to low; simmer, whisking constantly, until pudding thickens slightly, about 1 minute (it will thicken more as it chills). Remove from heat; stir in whiskey and vanilla. Transfer to bowl; cover with plastic wrap, pressing wrap directly onto surface of pudding. Chill at least 8 hours or overnight.
Make pie: Preheat oven to 375°F. Line interior of crust with parchment; fill with dried beans or pie weights, pushing beans or weights up against sides of parchment. Bake for 20 minutes. Remove parchment and weights, prick crust all over with a fork and bake until crust is deep golden brown, 10 to 15 minutes longer. Sprinkle chocolate in crust, let melt, then spread in a thin layer. Transfer to a rack and let cool completely.
Assemble pie: With an electric mixer at medium speed, beat cream with sugar and vanilla until it holds stiff peaks. Spoon pudding into crust, top with whipped cream, and garnish with chopped Heath bar. Refrigerate for 1 hour before serving.