Preparation / Directions:
FOR THE COOKIE CRUST:
Butter only the sides of an 8"x2.5" or 8"x3" Springform pan, not the bottom. The cookies must be ground to crumbs. Either break them into pieces and grind them in a food processor with a steel blade; OR grind them in batches in your blender; OR place in a heavy plastic bag and pound and roll them with a rolling pin until they are fine. You should have 1 cup of crumbs.
Mix the crumbs with the melted butter in a bowl, using a rubber spatula until the butter is evenly distributed. You will think there is not enough butter, but there is. Pour it into the baking pan and use your fingers to press it into a compact, even layer on the bottom only. Refrigerate
FOR THE CHEESECAKE:
Preheat the oven to 350F. Melt the chocolate (I do it in my microwave; it takes about 2 minutes on High, but I stop every 30 seconds to stir it to ensure it doesn't burn) and set aside to cool slightly. Beat the cream cheese until very smooth. Add the vanilla and sugar and beat to mix. Add the eggs one at a time, scraping the bowl with a rubber spatula and beating till smooth after each. Then add the salt and 1 1/2 cups of the sour cream and beat till smooth.
Beat the melted chocolate with the remaining 1/2 cup sour cream (you can use the same beaters you used for the cheese mixture without washing them). Remove 1 1/2 cups of the cream cheese mixture and beat into the chocolate.
Place the two batters, alternating colors, by spoonfuls over the crust in the pan. Use the flat side of a table knife to marbleize: cut down through the batters and use the knife to swirl them into large spirals and/or zigzags and form an attractive pattern. Don't over mix or you'll lose the contrast.
Briskly rotate the pan a bit first in one direction, then the other, to level the top. Bake 30 minutes on the lowest rack in your oven. It will seem soft, but it's done. Cool on a rack to room temperature, then chill at least 5-6 hours before serving c