Preheat oven to 350 degrees F. Line a 9x9-inch baking pan with aluminum foil and spray with cooking spray. Set aside.
Using an electric mixer, beat the cake mix, butter, and egg together on medium speed until a thick dough forms. Add in 1/3 cup of the sprinkles and mix on low just until incorporated.
Press about 2/3 of the dough into the bottom of the prepared baking pan. Sprinkle the white chocolate chips and the remaining 1/3 cup of sprinkles on top. Pour the sweetened condensed milk evenly over the top. Separate the remaining dough into small balls and evenly space over the top.
Bake for 30 minutes, or until they just begin to brown. Cool completely before slicing. Enjoy!