Preheat the oven to 350 degrees. Then line a 9×9 baking pan with parchment paper.
In the bowl of a stand mixer add in the cake mix, vegetable oil, milk, egg, vanilla extract, and sprinkles. Mix ingredients together until combined and smooth.
1 (15oz) box yellow cake mix, 1/4 cup vegetable oil, 3 Tbsp whole milk, 1 large egg, 1 Tbsp vanilla extract, 1/2 cup rainbow sprinkles
Using a rubber spatula, turn the batter out into the parchment lined baking pan. Spread the batter around evenly into the pan.
Place the baking pan into the preheated oven for 30 to 35 minutes. The cake is done when a toothpick inserted comes out clean.
When the cake is done baking, let the blondies cool at room temperature for a couple of hours before cutting into it for the best results.
Notes
Add mix-ins. peanut butter chips, white chocolate chips, butterscotch chips, cinnamon chips, dark chocolate chips, or milk chocolate chips. You could also drizzle melted white chocolate overtop or add cream cheese or classic cake frosting on top. Use sprinkles to suit the occasion. This works well with color-themed birthday parties! Again, I recommend using jimmies because the little ball sprinkles (also known as nonpareils) will bleed into the batter. Do not substitute other cake mixes for the yellow cake mix. This recipe works best with yellow cake mix.