Allow the stick of vegan butter to sit at room temperature for about 30 minutes until softened, but not overly soft.
Preheat the oven to 350°F and line two baking sheets with parchment paper.
In a large bowl, use a fork to mash the softened vegan butter. I do this by hand, but feel free to use a stand mixer or hand mixer on medium speed.
Add the brown sugar and mix until combined and creamed.
Stir in the plant-based milk, ground flaxseed and vanilla extract.
Toss in the all-purpose flour, old-fashioned oats, baking soda, cinnamon and salt and use a spatula to start combining. Once the mixture is almost combined, fold in the chocolate chips and continue mixing until no flour remains.
Using a cookie scoop, portion the dough into 20 pieces and roll into balls. If your dough feels very soft, I recommend refrigerating the dough for 15-30 minutes to firm up.
Place the first batch onto the prepared baking sheet, spacing them 2-3 inches apart, and refrigerate the remaining dough until the first batch has baked. Bake for 11-13 minutes, until the edges are golden but the center is still soft.
Remove the cookies from the oven and let them cool on the baking sheet for 5 minutes before transferring to a cooling rack.