Pre-heat the oven to 350ºF. Prepare a baking sheet with a silicone mat or parchment paper.
In a bowl mix flour, flax, oats, baking soda, and salt.
In a bowl of an electric mixer fitted with the paddle attachment mix coconut oil, peanut butter, sugar, vanilla, and milk. Mix until well integrated.
Add the dry ingredients to the wet ingredients. Be careful not to over mix. Add the dark chocolate chips.
With a small ice-cream scoop or tablespoon, drop 2 inch mounds of dough onto the prepared baking sheet pan.
Bake for 12 to 15 minutes until lightly golden. Transfer the cookies to a baking rack and cool completely.