Adjust the oven rack to the lower-middle position. And preheat the oven to 350°F (180°C,) or 160°C for a fan oven.

Line a 9x13-inch (23x33 cm) baking pan with parchment paper, letting the paper overhang on the sides so that the bars are easy to remove later. Set the pan aside.

Crush the Biscoff cookies using a food processor or blender until they are turned into fine crumbs. In a medium mixing bowl, mix together the crumbs, melted butter, and salt.

Add the crumb mixture to the prepared baking pan and press down firmly to cover the bottom of the pan in an even layer.

Bake in the preheated oven for 10 minutes. Remove from the oven and let cool on a wire rack. Leave the oven on.

Add the softened cream cheese and sugar to the bowl of your stand mixer (or use a hand mixer and a large mixing bowl). Beat on medium speed until smooth and creamy.

Add the cookie butter, sour cream, and vanilla extract and beat until well incorporated, scraping down the sides of the bowl with a rubber spatula a few times.

Then add the eggs, one at a time, mixing just until each egg is incorporated into the filling. Avoid overmixing.

Pour the filling into the crust and use a spatula to spread it out evenly. Bake at 350°F (180°C) for 30-35 minutes, or until lightly browned and the center of the cheesecake is set.

Allow the cheesecake to cool in the pan on a wire rack until it is cool to the touch. Then, transfer to the refrigerator and chill for 4 hours.

Microwave the cookie butter in a microwave-safe bowl for 30 seconds, then stir. If needed, microwave for an additional 15 seconds and stir again.

Pour the melted cookie butter over the chilled cheesecake and spread out evenly. Return the pan to the fridge to chill for 30 minutes, or until the topping is firm. Slice into squares to serve.