Preheat the oven to 350° and spray and line with parchment paper 2 half sheet pans (18×13-inch). In the bowl of a stand mixer or using a hand held mixer, mix together the boxed cake mix, corn starch, sugar, water, eggs, vanilla and oil until combined, about 1 minute. Add in the flour, salt and sour cream and mix on medium until incorporated and there aren't any lumps, 2-3 minutes.