Finely crush the Biscoff cookies into a fine crumb and mix with melted unsalted butter. Press into a 9-inch pie dish and bake at 350°F for 10-12 minutes. Allow to cool.

In a saucepan, combine granulated sugar and water over medium heat, stirring until sugar dissolves. Bring to a gentle boil for 4-5 minutes until deep amber color. Slowly whisk in heavy cream and remove from heat.

In another saucepan, whisk brown sugar, cornstarch, salt, and whole milk. Bring to a boil, stirring continuously, then mix in caramel sauce.

Beat egg yolks until smooth, gradually whisk in hot butterscotch mixture. Return to saucepan and cook until thickened, about 2-3 minutes. Stir in unsalted butter until melted.

Pour butterscotch mixture into cooled crust, smooth top, and cool at room temperature for 30 minutes. Cover with plastic wrap and chill in the refrigerator for at least 6 hours.

Whip heavy cream until soft peaks form, then add powdered sugar and vanilla bean paste. Spread over cooled butterscotch filling. Garnish with crushed Biscoff cookies or pecans if desired.