Preheat the oven to 350 degrees F. Place one oven rack in the middle of the oven, and one at the bottom. Place a large rimmed baking sheet on the bottom rack to catch any leakage. Line the bottom of a 9 1/2 inch springform pan with parchment paper. Then clamp the ring around the bottom tightly. (Trim the paper edges if desired.)
For the Crust: Place the Biscoff cookies, sugar, and salt in a food processor. Pulse until the cookies are finely ground. Then add the melted butter and pulse again to combine. Pour the crust mixture into the prepared pan. Press the crumbs with your hands over the bottom of the pan, and about 2/3 the way up the sides of the pan. Bake for 10 minutes.
For the Limoncello Cheesecake Filling: Meanwhile, place the cream cheese in the bowl of an electric mixer. Beat on high for 2 minutes to soften and fluff the cream cheese. One at a time add the eggs, not adding another egg until the previous egg is completely blended in. Scrape the bowl with a spatula. Then add the remaining ingredients and beat until completely smooth. Pour the filling into the crust and bake for 45 minutes.
For the Sour Cream Topping: In a medium bowl, mix the sour cream, sugar, and limoncello until smooth. Once the cheesecake has baked and is mostly set in the middle, spread the sour cream filling over the top of the cheesecake and place back in the oven for 10-12 minutes.
Cool the cheesecake on the counter for 1 hour. Then place in the refrigerator to chill and set for at least 3 hours. Later, take the ring off the springform pan and slide the cheesecake onto a cake plate. Slice and serve cold.