Step 1

Heat oven to 325°F.

Step 2

Line 13x9-inch pan with foil, with ends of foil extending over sides. Cook butter and chocolate in large saucepan on low heat until chocolate is completely melted and mixture is well blended, stirring constantly. Cool completely.

Step 3

Stir 1-1/2 cups sugar into chocolate mixture. Add 2 eggs, 1 at a time, stirring after each just until blended. Add milk and 1 tsp. vanilla; mix well. Gradually stir in flour; spread onto bottom of prepared pan.

Step 4

Beat Neufchatel, remaining sugar and vanilla in large bowl with mixer until blended. Add sour cream; mix well. Add remaining eggs, 1 at a time, mixing on low speed after each just until blended. Pour over chocolate layer in pan. (Batter will come almost to top of pan.)

Step 5

Bake 45 min. or until center is almost set. Cool. Refrigerate 4 hours. Use foil handles to lift cheesecake from pan before cutting to serve.