While it isn't required, using cake flour in this recipe does help create the perfect crumb on the finished coffee cake. If you don't have any cake flour on hand, don't worry. Here are two cake flour substitution options for you: 1. Remove 2 tablespoons of flour from every cup of flour and replace them with 2 tablespoons of corn starch. This is the preferred substitution option. 2. Remove 1 tablespoon of flour from every cup of flour and just use the all-purpose flour alone.