For crust, combine wafers and butter. Press onto the bottom of an ungreased 9-inch springform pan. Cut the split ladyfingers in half crosswise; line side of pan with ladyfinger pieces, rounded side out and cut side down. Set aside.
For filling, dissolve coffee powder or crystals in rum, brandy, or milk (theres no need to heat liquid to dissolve powder or crystals); set aside. Beat cream cheese and mascarpone until combined. Gradually add sugar, beating on medium to high speed until smooth. Beat in cornstarch and vanilla. Add eggs all at once. Beat on low speed just until combined. Stir coffee mixture into cheese mixture.
Pour filling into crust-lined pan. Place in a shallow baking pan in oven. Bake in a 350 degree F oven for 45 to 50 minutes or until center appears nearly set when gently shaken. Remove from oven. Immediately stir sour cream; gently spoon sour cream on top of hot cheesecake; carefully spread to within about 1 inch of edge.
Cool in springform pan on wire rack for 15 minutes. Use a narrow metal spatula to carefully loosen ladyfingers from side of pan. Cool 30 minutes more. Remove sides of pan; cool 1 hour. Cover and chill at least 4 hours before serving. To serve, sift cocoa powder over top and, if desired, sprinkle with shaved chocolate. Makes 12 to 16 servings.