<p>According to the Cake doctor web site you can safely add a 1/2 cup of flour to a packaged cake mix. From my baking experience I would say that cookie mix would be just as forgiving. If you want to cut the sweetness a bit more by putting in around a cup of flour, you also need to add a proportional amount of shortening/oil/butter, at least a tablespoon, 2 tablespoons of water and I’d say a half teaspoon of baking powder.</p>