Prep oven + baking sheet: Preheat oven to 350 degrees F and line a cookie sheet with recycled parchment paper.
Mix wet ingredients: In a medium bowl, mix together ground flaxseed meal and room temperature water to make a flax egg, and add chia seeds. Let sit for 5-7 minutes to allow mixture to thicken (it will become your egg-free binder), then add melted vegan butter (or coconut oil, if using), vanilla extract, and brown sugar and whisk together until well combined.
Mix dry ingredients: In a large mixing bowl, add rolled oats, flour, baking soda, and salt and mix until well combined.
Combine wet + dry ingredients: Add wet ingredients to dry ingredients and mix until well combined. The cookie batter should be thick, wet, and hold together well, not runny. If needed, at a bit more rolled oats or water until desired consistency. Finally, fold in chocolate chips and mix until well dispersed throughout batter.
Chill batter: Refrigerate cookie batter for about 15-20 minutes to cool the batter and produce a thick and chewier cookie, which allows the ingredients to stay sticky when baked. You can skip this step and simply bake, but you'll have a thinner and crispier cookie.
Scoop cookies: Next use a medium spoon or cookie scoop and form cookie dough into 2 tablespoon heaps on prepared baking sheet, placing cookies about 2 inches apart. For thinner cookies, gently flatten dough so cookies bake outward.
Bake cookies: Bake cookies for about 10 minutes, or until the edges begin to brown. If cookies are still very soft on the sides when touched with a fork, bake another 1-2 minutes. However, slightly under-baking is suggested as the cookies will continue to harden once removed from the oven.
Cool cookies: Once baking sheet is removed from the oven, let cookies cool directly on the cookie sheet to allow them to cool and hold together. Once cooled completed, store in an airtight container for up to four days.