Preheat oven to 350 degrees F and grease an 8x8 baking pan with nonstick cooking spray. Set aside.
Add cake mix, oil and eggs in a bowl and mix well by hand. The batter will be very stiff so use a strong spoon! Continue to mix until combined well. Pour batter into prepared pan.
Bake for 25-30 minutes or until center is set. Brownie will be puffy and flatten as it cools.
Cool completely.
Once cooled, make frosting. Add chocolate chips to a microwave safe bowl and heat at 50% power for 1 minute. Stir and repeat, stirring until smooth. Add sweetened condensed milk and mix well. If mixture isn't smooth, return to the microwave for 30 seconds on 50% power and mix until smooth.
Pour on top of brownies and spread evenly. Quickly sprinkle with sprinkles as icing begins to set up quickly. Let stand to cool completely before cutting into 9 bars.