To make chocolate sauce
Whisk together the dry ingredients in a bowl.
Heat the milk, vanilla and butter over medium heat while whisking constantly until the butter melts.
Whisk in the dry ingredients.
Keep whisking until the mixture boils. Whisk for about a minute longer and remove from heat.
Let cool. Store extra sauce not used on the ice cream pie in the refrigerator.
To make the pie
Place the entire box of Thin Mints and melted butter inside a food processor and pulse until the crumbs bind together. Alternatively, crush the Thin Mints manually and then mix with melted butter.
Press the crumb mixture into an 8" springform pan or pie dish. Try to press it one inch up the sides though if using a springform pan, don't worry about it.
Scoop the softened ice cream into the crust. You'll use the entire gallon and could even add in more, if using a springform pan.
Use an angled spatula to smooth out the top. It helps to run it under hot water first.
Pour your desired amount of chocolate sauce on top. Make sure that it isn't hot, otherwise it will melt the ice cream.
Let the completed pie sit in the freezer for about 4 hours. Cut and serve.