This is a not-so pretty side effect of baking with sunflower butter (but kids think it's so fun!). In short, sunflower butter contains chlorophyll (from the sunflower seeds) which, when baked with baking soda, can result in a dark green color once cooled. These oatmeal chocolate chip cookies don't turn as green as most sunflower butter cookies though! Either way, this side effect is harmless!