Preheat oven to 350 degrees. Combine Biscoff crumbs, melted butter, and brown sugar in a bowl and stir so that crumbs and sugar are well saturated with butter.
Press into bottom of a 9x9 square baking pan, pressing down with fingers or the bottom of a dry measuring cup.
Bake crust in preheated oven for 8 minutes. Let cool slightly.
For the Cheesecake Layer
Reduce oven heat to 325°F.
Beat cream cheese on medium speed in the bowl of a stand mixer (you can also use a hand-held mixer) until smooth and creamy (do not overbeat). Add sour cream and beat until smooth, scraping sides of bowl to fully incorporate everything.
Add sugar, vanilla and salt and beat until smooth.
Beat in egg until smooth. Remove ½ cup of cheesecake filling and set aside.
Pour the rest of cheesecake filling into prepared crust and smooth to cover the crust evenly.
For the Pumpkin Pie Layer
In a bowl, combine pumpkin, sugar, heavy cream, vanilla, spices and salt and mix well.
Beat in egg until smooth. Pour over the cheesecake layer and spread evenly.
For the Swirl Topping
Drop spoonfuls of the reserved cheesecake filling all over the pumpkin pie layer.
Use a toothpick, knife, or dowel to swirl the pumpkin and cheesecake layers together to create a marble pattern.
Bake bars in preheated oven for 45-50 minutes, or until center is set and only slightly wobbly.
Remove from oven and allow to cool fully at room temperature.
Cover with plastic wrap and chill in refrigerator for 4 hours or overnight before slicing into bars.
Pumpkin pie spice: The cinnamon, nutmeg, ginger, and cloves work as a homemade pumpkin pie spice. If you have a store-bought pumpkin pie spice on hand, feel free to use 1 teaspoon of that instead.Storing and freezing: Because of their cheesecake layer, these bars should be kept refrigerated until they are served. I like to place the sliced bars in a rectangular airtight food container and store them that way. They will keep, refrigerated, for up to 3 days. You can also freeze sliced bars for up to 3 months – just place the sliced bars on a baking sheet and freeze for about an hour uncovered, then transfer to a freezer-safe airtight container.