I forgot to add that you can substitute up to 1/4 cup of cocoa for 1/4 cup of the flour without making any other changes.
Dutch process cocoa works best but you can also use natural cocoa.
I have made them with this substitution and also with adding just a tablespoon (15 ml - American tablespoon) of powdered espresso coffee. You can use instant coffee but if it is the crystalline type you have to crush it to a fine powder.
You don't really taste the coffee, it enhances the cocoa flavor.