Preheat the oven to 350°F. Line a large baking sheet with parchment paper.
Make your flax egg: In a small bowl, whisk together 2 tablespoons of ground flax seeds with 4 tablespoons of room temperature water. Set aside and let it thicken while you prepare the recipe.
In a medium or large bowl, combine your rolled oats, flour, baking soda, salt and cinnamon. Set aside.
In a different medium bowl, beat together the butter and sugar. Stir in the maple syrup, flax egg and vanilla until combined.
Transfer the wet mixture to the dry mixture and combine. Don't overmix! Fold in the chocolate if using.
Form 10 cookies and place them on the prepared baking sheet, leaving some distance between each cookie. They do spread. Form a little mound with each cookie. This limits the spread and makes thick and chewy cookies (see photos).
Bake for 15 minutes at 350°F. Let them cool for at least 5-10 minutes on the baking sheet. They will be delicate when warm. Enjoy!