Preheat oven to 350 F. Spray a cake pan with non stick spray.
In a small bowl, combine flour, potato starch, salt and baking powder.
In the bowl of an electric mixer, cream butter and sugar. Add vanilla, zest and Cognac and mix well. Add eggs separately, beating well after each addition.
Add flour combination in 3 parts, beating well after each addition.
Transfer to prepared baking pan and bake for 30-40 minutes until toothpick inserted comes out clean.
Cool before slicing.
Notes
You can substitute other starch products like cornstarch, arrowroot or tapioca starch for the potato starch.