How to Make It
Preheat oven to 325°F. Wrap outside bottom and sides of 9-inch springform pan with foil to prevent leaking. Lightly spray inside bottom and side of pan with cooking spray. Combine crumbs and melted butter in a medium bowl. Press in bottom and halfway up sides of pan. Bake at 325°F for 10 minutes or until set. Cool crust 10 minutes.
Beat cream cheese, sugar, and vanilla at medium speed with an electric mixer until smooth. Beat in eggs, 1 at a time, just until blended after each addition. Divide batter evenly between 2 bowls. Beat melted chocolate into 1 bowl; stir in 3/4 cup of the sour cream. Beat remaining 1/4 cup sour cream into second bowl; stir in chopped toffee candy bars. Pour toffee batter over crust. Carefully spread with chocolate batter.
Bake at 325°F for 1 hour and 15 minutes or until almost set. Turn oven off. Let cheesecake stand in oven, with oven door open at least 4 inches, for 30 minutes. Remove from oven, and gently run a knife around outer edge of cheesecake to loosen from sides of pan. (Do not remove sides of pan.) Cool on a wire rack 30 minutes. Refrigerate at least 6 hours or overnight.
Just before serving, run small metal spatula around edge of pan; carefully remove foil and sides of pan. Set oven control to broil. Place cheesecake on a baking sheet. Cut marshmallows in half horizontally with dampened kitchen scissors. Place marshmallows, cut side down, on top of cheesecake. Broil about 6 inches from heat 1 to 2 minutes or until golden brown.