Preheat oven to 450 degrees F. Grease a 10-inch spring form pan.
If needed, use a rolling pin with the cookie crumbs in a sealable plastic bag to crush them. Blend the cookie crumbs with the melted butter, press it into the bottom of the pan, and chill.
In a mixer with a whisk attachment, place the softened cream cheese and whip it. Add the sugar and continue to mix until blended. Add the flour and salt and blend. Add the yolks and eggs gradually, scraping in between then add the vanilla. Add 3/4 cup of the sour cream and mix in. Take off 2 cups of the mixture and set aside.
In a small bowl, whisk the remaining 1/4 cup sour cream with the cooled melted chocolate then whisk it into the reserved 2 cups of filling. Pour a layer of the vanilla filling into the bottom of the pan about 1-inch thick. By large spoonfuls, alternate the 2 fillings to fill the spring form pan. Using the flat face of an icing spatula, swirl the 2 batters slightly to marbleize it.
Bake at 450 degrees F for 15 minutes, then turn the oven down to 250 degrees F and bake for 1 more hour. Then, turn the oven off and let it cool in the oven 1 hour. The cake will still shimmy, but it is done, it will set up as it chills. Refrigerate the cake overnight. Remove from the spring form pan, slice with a hot, dry knife, and serve.
Notes about the recipe: I had to make 6 cheesecakes a day when I was pastry chef at the Strathallen Hotel in Rochester New York. Just to rotate them halfway through the baking was a major production. I didn't have may own baking oven so had to use the ones on our very busy hot line. It was a trick just to keep the guys (cooks) from changing the temperatures on my ovens because they always last them to 500. And then to get them to move so I could kneel in front of the ovens to juggle the cakes around...forget it. So I'd have to yell out "ROTATE!" and get everyone away from the oven while I kneeled in front of them to do the rotation ballet of hot shimmying cheesecakes with dry towels (a hot commodity in kitchens). My knees on my chef's pans were always dirty and that was why.