Preheat oven to 350° and spray 8x8 pan with cooking spray.

Mix box of yellow cake mix, egg, 1/4 cup of sprinkles, and softened butter with electric mixer until soft dough is formed

Press half of the dough into the prepared 8x8 pan

Pour the sweetened condensed milk over the dough. Then sprinkle the white chocolate chips and remaining sprinkles on top

Take the remaining dough and roll into small balls

Distribute balls evenly over the condensed milk then press down to flatten (Don't worry about the dough not evenly/completely covering the condensed milk)

Bake dessert bars for 30 minutes, or until golden brown. Let completely cool before cutting otherwise it will be far too gooey.