Preheat oven to 350°F. Using 9-inch springform pan, combine crumbs and butter. Press firmly onto bottom of pan.
Beat cream cheese and sugar in large bowl with mixer on medium speed until blended. Add sour cream and eggs one at a time, beating on low speed after each addition just until blended. Chop 8 squares of chocolate; stir into batter. Pour over crust.
Bake 45 to 50 min or until center is almost set. Run a knife or metal spatula around side of pan to loosen cake. Cool completely.
Chop remaining 4 squares of chocolate. Bring cream to simmer in small saucepan on low heat. Remove from heat. Add chocolate; stir until completely melted. Cool slightly. Pour over cheesecake. Refrigerate at least 3 hrs or overnight. Remove sides of pan before serving. Store in refrigerator.