Preheat oven to 350°. Line a 9x9” pan with foil and spray liberally with cooking spray.
Add cake mix, butter, and egg to the bowl of an electric mixer fitted with a paddle attachment. Mix until a thick dough forms.
Press about 2/3 of the dough into the bottom of the prepared pan. Sprinkle with white chocolate chips.
Make sure your cream cheese is very soft, even almost melted. You can heat it in a microwave safe bowl in 20 second increments in your microwave until you’re able to whisk it. Whisk the cream cheese with the sweetened condensed milk. Pour sweetened condensed milk mixture over the white chocolate.
Separate remaining dough into small balls and evenly space over the top of the bars. Press down to flatten. It will not cover completely. Sprinkle with sprinkles, if desired. Note: don’t use nonpareils, they’ll melt. Use jimmies instead.
Bake bars for about 28-30 minutes, or until they just begin to brown. Cool completely before slicing, otherwise the mixture will be too gooey to cut.
Store in an airtight container for up to 3 days or freeze for up to one month.