Prepare the Crust: Begin by crushing the Biscoff cookies in a food processor until fine. In a bowl, mix the crushed cookies with melted butter and sugar until combined. Press the mixture firmly into the bottom of a springform pan to create a solid base.

Make the Filling: In a large mixing bowl, beat the softened cream cheese and sugar together until smooth and creamy. Gradually add in the eggs, one at a time, mixing well after each addition. Then, fold in the sour cream, pumpkin puree, pumpkin pie spice, and vanilla extract until everything is well combined.

Bake: Carefully pour the creamy filling over the prepared crust in the springform pan. Bake the cheesecake at 325°F (163°C) for approximately 50-60 minutes, until the edges are set but the center still has a slight jiggle.

Cool: After baking, let the cheesecake cool at room temperature for about an hour. Then refrigerate it overnight to let the flavors meld and the texture set beautifully.

Prepare Whipped Marshmallow Cream: In a separate bowl, whisk together egg whites and sugar until stiff peaks form. Then, add cream of tartar and vanilla extract for flavor. The result should be a light, fluffy cream that's perfect for topping.

Serve: Once the cheesecake has chilled thoroughly, remove it from the springform pan. Top it generously with the whipped marshmallow cream just before serving.