When making green cookies, spinach is an obvious choice. It adds colour, nutrition, and doesn't alter the flavour of the food.
I've been having a lot of baking failures as of late, I needed something simple to make that didnt involve flour.
This recipe also doesn't take very long to make. It makes 5 small cookies, it's easy and doesn't make a mess. I never bake huge batches of desserts, but if you want more you can double or triple the recipe easily.