Preheat oven to 350°F (177°C). Line an 11×7-inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out of the pan. Or skip the parchment and spray with nonstick spray. Set aside.

Combine the cake mix, oil, egg, and milk in a large bowl with a silicone spatula. The batter is extremely thick, so do your best to mix. Fold in the sprinkles and white chocolate chips.

Spread evenly into prepared baking pan, sprinkle a few more sprinkles on top (if desired).

Bake for 25-30 minutes, or until the edges are lightly browned. The center of the pan will still look soft; that’s OK. Avoid over-baking.

Remove from the oven and cool the pan for 30 minutes at room temperature on a wire rack.

Lift the parchment paper out of the pan and cut into squares.

Blondies stay fresh in an airtight container at room temperature for up to 1 week.