Preheat oven to 350°F. Grind the graham crackers in a food processor until fine. Add sugar, melted butter, and salt and pulse until mixed. Press into the bottom of a springform pan. Wrap the pan in tinfoil (I double wrap mine) and set in a rimmed baking sheet.
Cook sugar in a dry heavy saucepan over moderately low heat, stirring slowly with a fork, until melted and pale golden. Continue to cook until deep golden. Remove from heat and carefully add heavy cream (mixture will vigorously steam and lumps may form, this is okay). Return to moderately low heat, stirring, until caramel lumps are dissolved. This may take a few minutes, but it will happen. Remove from heat and whisk in chocolate until smooth. Stir in sour cream.
Beat cream cheese with an electric mixer until fluffy, then beat in chocolate mixture on low speed. Beat in eggs, one at a time, then vanilla, beating on low speed until each ingredient is incorporated and scraping down bowl between additions.
Pour filling into crust and place in the middle of the oven. Fill a large pyrex with water and pour it gently into the rimmed baking sheet until water goes at least an inch up the side of the foiled springform. Bake 50 - 60 minutes, or until cake is set 3 inches from edge but center is still slightly wobbly when pan is gently shaken. Turn off oven and prop door open until the cake and oven come down to room temperature.
Remove cake from oven and run a knife around the edge. Chill and then gently remove springform sides. Continue chilling or freeze until ready to eat.