Preheat oven to 350 degrees F. In a large bowl, combine the yellow cake mix, egg, milk and oil. Stir in white baking chips and jimmies. Spread into a greased 15x10x1 - inch baking sheet.
Bake 18 - 20 minutes or until a toothpick inserted in the center comes our clean. Cool completely in pan on a wire rack. Cut into bars.
To freeze the bars: Freeze in freezer containers. Thaw in covered containers before serving.
Notes
The number of bars you get will depend on the size you cut them.