Preheat oven to 350°. 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. Bake 13 to 15 minutes or until lightly browned.
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 and vanilla, beating just until blended. Pour over baked crust.
Bake at 350º for 25 minutes or until set. Cool completely on a wire rack (about 1 hour). Cover and chill 4 to 24 hours; cut into bars.
If desired, spoon frosting into a zip-top plastic freezer bag. (Do not seal.) Snip 1 corner of bag to make a small hole. Pipe a frosting monogram on each bar. Garnish, if desired.