Preheat oven to 350.
Line a 9x9” pan with foil and spray with non-stick spray.
Stir cake mix, butter, and egg until a thick dough forms.
Add 1/3 cup sprinkles and stir until well distributed.
Press 1/2 of this mixture into the bottom of the pan.
Top with white chocolate chips and the other 1/3 cup sprinkles.
Pour condensed milk over the top.
Break remaining dough into small pieces and place evenly over the top of the bars. Press and flatten to form a complete layer.
Bake 30 minutes or until top begins to brown.
Cool completely before slicing.