Preheat oven to 350 degrees F and grease an 8 ½" x 11" or 8" x 12" baking dish.
In a large bowl, mix together the cake mix and ¼ cup of sprinkles.
Add the oil and egg and stir to combine. Then add the heavy cream and vanilla, and stir until combined.
Add the white chocolate chips and stir until chocolate chips are well distributed.
Spread into the pre-greased baking dish and level out with a spoon.
Sprinkle the top with the additional 1 tablespoon of sprinkles.
Bake for 30-33 minutes until a toothpick comes out clean.
Cool completely (possibly overnight), then cut into small squares.
Notes
Let the cake bars set and cool before cutting! I like to cool them overnight for this reason. If you try to cut them too early, they WILL fall apart on you.
You could use funfetti cake mix, but I like using separate sprinkles so I can customize the colors and then also have sprinkles to add on top.