Mix base cake, 3/4 cup of the sugar and butter until well blended. Divide mixture among 3 (9-inch) springform pans; press mixture firmly onto bottom of each pan. Bake in a preheated 325 degree oven for 8 minutes. Cool.
Place cream cheese and remaining sugar in bowl of electric mixer fitted with paddle attachment. Beat at low speed until well blended. Add sour cream and vanilla; beat until well blended, occasionally scraping down side and bottom of bowl. Gradually add eggs, mixing on low speed after each addition just until blended. In medium bowl melt chocolate until 110 degrees F. Add 8 oz of batter to melted chocolate and stir quickly to combine. Add chocolate mixture to cheesecake batter and stir until uniform in color.
Stir OREO pieces into batter.
Bake at 325°F for 1 hour 10 minutes or until centers are almost set. Run knife or metal spatula around rims of pan to loosen cakes; cool completely before removing rims of pan. Refrigerate at least 3 hours before serving.