Preheat oven to 350 degrees F.
Blind bake the pie shell. Set aside to cool.
In a small saucepan, melt the butter over medium heat. Add the chopped chocolate and remove from the heat. Mix until the chocolate is melted.
Whisk in the sugar, eggs, liqueur, flour, salt, and vanilla. Pour the filling into the pie shell. Bake for 28 to 30 minutes, or until the chocolate moves slightly. Let cool completely on a rack at room temperature.
Serve the pie by itself, topped with whipped cream.
To turn it into an amazing milkshake:
Chill the pie until firm. Cut into 8 slices. For each shake, in a blender, add 1 slice of the pie, 1/4 cup whole milk, 5 ounces ice cream, and 1 full-size Peppermint Patty. Turn on the blender and let it rip. Voila! Awesome milkshake. Serve topped with whipped cream and a mini Peppermint Patty.