1Heat oven to 325°F. Wrap outside bottom and side of 9-inch springform pan with foil to prevent leaking. Lightly spray inside bottom and side of pan with cooking spray. In medium bowl, mix crust ingredients. Press in bottom and halfway up side of pan. Bake 10 minutes or until set. Cool crust 10 minutes.
2Meanwhile, in large bowl, beat cream cheese, sugar and vanilla with electric mixer on medium speed until smooth. Beat in eggs, one at a time, just until blended. Divide batter 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. Pour toffee batter over crust. Carefully spread with chocolate batter.
3Bake 1 hour 15 minutes or until edge of cheesecake is set at least 2 inches from edge of pan but center of cheesecake still jiggles slightly when moved. Turn oven off; open oven door at least 4 inches. Let cheesecake remain in oven 30 minutes. Run small metal spatula around edge of pan to loosen cheesecake. Cool in pan on cooling rack 30 minutes. Refrigerate at least 6 hours or overnight.
4Just before serving, run small metal spatula around edge of pan; carefully remove foil and side of pan. Set oven control to broil. Line cookie sheet with cooking parchment paper; spray paper with cooking spray. Cut marshmallows in half horizontally with dampened kitchen scissors. Place marshmallows, cut sides down, on cookie sheet. Broil about 6 inches from heat 1 to 2 minutes or until golden brown. Transfer marshmallows to top of cheesecake with spatula. Store covered in refrigerator.