1. Position rack in the center of the oven and preheat to 350 F. Butter or coat w/non-stick spray the bottom of springform pan. Line a baking sheet with parchment paper. Set aside.

2. To make the crumble: sift together the flour, cocoa, sugar and salt and transfer to bowl of food processor with metal blade. Process on and off until blended. Arrange chunks of butter around the flour mixture and process until mixture resembles coarse meal. [NOTE: Mom doesn't have a food processor, so I just put it all into a bowl and used two sharp knives to cut the butter into the flour/cocoa--like you do with a crumble. It worked fine.)

3. In a small cup or bowl, whisk together the egg yolks and cream. With the processor running, pour the mixture through the feed tube making certain you scrape all the liquid from the cup. Let the machine run until the ingredients come together to form a dough, about 1 minute. You should have about 4 cups of this crumble mixture.

4. To make the cheesecake's crust: spoon 2 cups of the crumble mixture into the prepared pan and press evenly over the bottom. Use the bottom of a dry measuring cup to help you level the crust and give it smooth edges.

5. Spread the remaining crumble on the prepared baking sheet. Place the springform pan and baking tray in the oven and bake until crumble is lightly toasted, 10 to 12 minutes. Turn off the oven. Transfer both sheet and pan to wire rack to cool completely. When the springform pan is cool, wrap heavy-duty aluminum foil or 2 layers of regular foil around its bottom and halfway up the outside, pleating the foil to tighten it securely. Set aside. Using your hands or a fork, break up the crumble on the baking sheet into small chunks and set aside.

6. To make the filling: put the cream cheese, sugar and salt in the large bowl of an electric stand mixer or a large mixing bowl. Beat ingredients on medium speed until smooth, scraping down bowl with rubber spatula. Turn the speed to high and continue to beat until the mixture is creamy. Stop the mixer and add sour cream, eggs, and vanilla; beat 3 minutes longer, again stopping as necessary to scrape down sides of bowl under the blade.

7. Put pieces of white chocolate in microwave-proof cup or bowl. Put container in microwave, cover with paper towel, and heat for 30 seconds. Stop and stir the chocolate. Repeat process once or twice more just until the chocolate is completely melted and smooth.

8. Stir the melted chocolate into the cream cheese mixture and continue to beat until well combined and smooth. You should have 6 cups of the mixture.

9. Preheat the oven again to 350 F. Bring kettle of water to boil. Pour half cream cheese mixture into springform pan. Sprinkle half the chocolate crumble over the mixture. Pour in remaining cream cheese mixture, smooth its top with a rubber spatula and then scatter the remaining chocolate crumble evenly over its surface.

10. Place the springform pan into a slightly larger baking pan. Using an oven glove, pull out the oven shelf and place the baking pan on it. Pour enough hot water into baking pan to reach halfway up the sides of hte springform pan but not above the foil. Carefully slide the shelf into the oven and bake the cheesecake for 1 hour, checking the water level after about 30 mins and topping it up if necessary. Cover the cake loosely with foil and conitnue to bake until its center is slightly firm, about 20 mins longer.

11. Carefully remove baking pan from the oven. Lift out the springform pan and place on a wire rack to cool, carefully folding down the foil or its sides to promote quicker cooling. When the pan is cool enough to touch, completely remove foil and continue cooling. When the cheesecake is completely cool, cover pan loosely with clean sheet of foil and refrigerate overnight.

12. When ready to serve, remove the cheesecake from the fridge. Dip a long sharp knigfe in warm water and run knife around inside of springform pan--remove outer ring. Continue to dip knife in warm water to cut neat wedges.