Substitution for Cake Flour: Using all purpose flour (plain in the UK) for each cup of flour in a recipe remove 2 Tablespoons flour and replace with 2 Tablespoons cornstarch (corn flour). This recipe has 3 cups of flour so you will measure out 3 cups of all purpose flour, remove 6 Tablespoons and replace with 6 Tablespoons of cornstarch. Whisk to blend.