Prepare the Crust: In a food processor, pulse the Biscoff cookies until they form fine crumbs. Add melted butter and pulse again until the mixture holds together in clumps. Press the crumb mixture evenly into a pie pan, going up the sides. Refrigerate while you prepare the filling.
Make the Filling: Set aside the heavy cream. In a medium saucepan, whisk together brown sugar, water, and salt over medium heat. Bring to a boil without stirring and cook for about 5 minutes until the mixture darkens and smells caramelized.
In a separate bowl, whisk together egg yolks, cornstarch, cinnamon, vanilla, and 1 tablespoon of the milk. Set aside the remaining milk.
Once the sugar mixture is caramelized, reduce the heat to medium-low. Slowly pour in the heavy cream, whisking continuously until smooth (the sugar may clump, but keep whisking). Gradually add the remaining milk.
While stirring, bring the mixture to a simmer. Remove 1/2 cup of the hot mixture and slowly whisk it into the egg mixture to temper the yolks. Carefully pour the tempered egg mixture back into the saucepan, whisking constantly.
Cook over low heat, whisking continuously, until it starts to bubble and thicken. Continue whisking for an additional 1 1/2 minutes to ensure the filling sets properly. Stir in the butter until melted.
Assemble the Pie: Remove the crust from the fridge and pour in the butterscotch filling. For an extra smooth filling, strain the mixture through a sieve. Cover the filling with plastic wrap, ensuring it touches the surface to prevent a skin from forming. Refrigerate for at least 6 hours, or overnight.
Prepare the Whipped Cream Topping: When ready to serve, whip the heavy cream, powdered sugar, and vanilla in a medium bowl until soft peaks form. Spread over the chilled pie and dust with cinnamon.