Combine graham cracker crumbs, chopped walnuts, 2 tablespoons sugar, and butter, and mix well; firmly press mixture onto bottom and sides of an 8-inch springform pan.
Break candy bar into several pieces, and place in top of a double boiler; bring water to a boil.
Reduce heat to low, cook until chocolate melts.
Set chocolate aside.
Beat cream cheese with an electric mixer until light and fluffy.
Combine 3/4 cup sugar, 2 tablespoons cocoa, and dash of salt, gradually add to cream cheese, mixing well.
Add eggs, one at a time, beating well after each addition.
Add melted chocolate, beat until blended.
Stir in 1/2 cup sour cream and 1/2 teaspoon vanilla, blending well.
Pour into prepared pan.
Bake at 325° for 40 minutes.
Turn off oven, and let cheesecake stand in closed oven 30 minutes.
Let cheesecake cool to room temperature, then chill.
Remove sides of springform pan, spread with Sour Cream Topping, and sprinkle on walnuts.