Prepare the Crust: Preheat the oven to 350°F (175°C) and grease a 9-inch springform pan. Mix the crushed Biscoff cookies with the melted butter until well combined. Press the mixture evenly into the bottom of the pan to form a firm crust. Bake for 10 minutes, then set aside to cool.

Make the Cheesecake Filling: In a large mixing bowl, beat the softened cream cheese and brown sugar until smooth and creamy. Add the pumpkin puree, sour cream, vanilla extract, and pumpkin spice. Mix until well combined. Beat in the eggs and egg yolks one at a time, ensuring each is fully incorporated. Sift in the cornstarch and mix until smooth.

Assemble the Cheesecake: Pour the cheesecake filling over the prepared crust and smooth the top with a spatula. Place the pan in a large roasting pan and fill the roasting pan with hot water, creating a water bath to prevent cracking.

Prepare the Streusel Topping: In a medium bowl, mix the flour, powdered sugar, granulated sugar, and pumpkin spice. Cut in the cold butter with a pastry cutter or fork until the mixture resembles coarse crumbs. Sprinkle the streusel evenly over the cheesecake batter.

Bake the Cheesecake: Bake the cheesecake at 325°F (160°C) for 1 hour and 15 minutes, or until the center is slightly set but still jiggles. Turn off the oven, crack the door slightly, and let the cheesecake cool inside for 1 hour. Refrigerate the cheesecake for at least 4 hours or overnight before serving.

Garnish (Optional): Whip the heavy cream, powdered sugar, and vanilla extract until soft peaks form. Pipe or dollop the whipped cream onto the cheesecake before serving for an elegant touch.