Preheat the oven to 350°F and prepare 2 half sheet pans (18×13-inch) with parchment paper.
Mix the boxed cake mix, sugar, water, eggs, vanilla, and oil until combined.
Add flour, salt, and sour cream, mixing until smooth (2-3 minutes).
Divide the batter into both pans and bake for 15 minutes or until fully cooked.
Cool at room temperature for 30 minutes, then refrigerate for 30-60 minutes.