Prepare cake batter according to package directions. Pour into a greased 15" x 10" baking pan.
In a small mixing bowl, beat cream cheese and sugar. Stir in 1/4 cup chocolate chips. Drop by tablespoonfuls over batter. Cut through batter with a knife to swirl the cream cheese mixture. Sprinkle with remaining chocolate chips.
Bake at 350 degrees for 25-30 minutes or until a toothpick inserted near the center comes out clean. Cool on a wire rack. Cut into bars.