Mix crumbs, butter, sugar, and cocoa and press into greased 8" springform pan.
Mix cream cheese, sugar, vanilla, eggs, and sour cream. (To marble, save a little and add cocoa.)
Pour into pan.
Bake at 350 for 45-70 minutes. (Sprinkle with chocolate chips after 15-30 minutes.)
Turn off oven but do not remove cake; leave ajar for 1 hour.
Remove cake and cool.
Chill at least 6 hours.