Preheat the oven to 350°F and grease a 9x9-inch baking pan or line it with parchment paper and set aside.
Mix together the cake mix, vegetable oil, milk, egg, and vanilla extract. The batter should be well mixed but stiff.
Transfer batter to the prepared baking pan and spread evenly with a rubber spatula. Bake for 30 to 35 minutes until a toothpick comes clean from the center of the blondies.
Blondies should rest for a couple of hours before cutting for best results.
Notes
This recipe will work with any cake mix flavor if you want to mix it up.
Add a layer of frosting for a true cake-like experience.