Preheat your oven to 350 degrees F. Grease a 9-inch x 9-inch baking pan or line it with parchment paper and then set it aside.
Mix together cake mix, vegetable oil, milk, egg, and vanilla extract in a large bowl. The batter should be well mixed but stiff. Mix in the sprinkle with a rubber spatula or wooden spoon. (Save some sprinkles for topping.)
Transfer batter to the prepared baking pan and spread into an even layer with a rubber spatula. Sprinkle the reserved sprinkles over the batter.
Bake for 30 to 35 minutes until the toothpick comes clean from the center.
Remove from the oven. Blondies should rest for a couple of hours before cutting into 9 bars for best results.
Notes
I do NOT recommend using funfetti or white cake mix for this recipe, only yellow.