1. Beat first 3 ingredients at medium-low speed with an electric mixer until combined. Increase speed to medium, and beat until well blended and crumbly. Pat mixture into a lightly greased 13- x 9-inch pan.
2. Bake at 350° for 13 to 15 minutes or until lightly browned.
3. Beat cream cheese at medium speed with an electric mixer until creamy. Gradually add granulated sugar, beating until well blended. Add eggs, 1 at a time, beating at low speed just until blended after each addition. Add sour cream, vanilla, and 1 cup chocolate morsels, beating just until blended. Pour over baked crust.
4. Bake at 350° for 25 minutes or until set. Cool completely on a wire rack.
5. Microwave remaining chocolate morsels in a 2-cup glass measuring cup on HIGH 1 minute, stirring after 30 seconds. Stir until smooth. Cool slightly. Drizzle over cheesecake. Cover and chill at least 4 hours; cut into bars.