Preheat your oven to 350°F (175°C). Wrap a 9-inch springform pan in foil to prevent water from seeping in.

In a food processor, pulse the Biscoff cookies until finely crushed. Combine with melted butter and granulated sugar, then press firmly into the bottom of the prepared pan. Bake for 12 minutes until set and lightly golden, then cool.

In a stand mixer, beat the cream cheese until smooth. Add the granulated sugar and cornstarch, mixing for 2-3 minutes on medium speed.

Add the canned pumpkin puree, sour cream, pumpkin pie spice, and vanilla extract, mixing until combined. Avoid overmixing.

Beat in the eggs and egg yolks one at a time at low speed, allowing each addition to blend in completely before the next.

Carefully pour the filling into the cooled Biscoff crust. Place the springform pan inside a larger roasting pan, pouring boiling water into the roasting pan until it reaches about 1.5 inches up the pan's sides.

Bake for 1 hour. Turn off the oven, crack the door open, and let cool for an additional hour.

Run a butter knife around the edges of the cheesecake to loosen it. Allow to cool to room temperature, then refrigerate for at least 8 hours or overnight.

To prepare the frosting, beat soft butter and cream cheese together until well blended. Gradually add powdered sugar until light and fluffy.

Once chilled, remove the cheesecake from the springform pan and spread the frosting around the edges. Sprinkle extra crushed Biscoff cookies on top before serving.