Preheat the oven to 350 degrees Fahrenheit. Line a large baking sheet with parchment paper.
In a medium bowl, add oats, flaxseed, coconut sugar, cocoa powder, and salt. Whisk to combine. Add mashed banana, peanut butter, vanilla extract, coconut oil, and almond milk. Stir until mixture is evenly combined. If using, fold in chopped walnuts and/or dairy-free chocolate chips.
Using a tablespoon, scoop cookie dough and place on baking sheet in even rows. Use the palm of your hand (use a piece of parchment for less mess) to press cookie down slightly. The cookies won't spread more than they are pressed.
Bake for 25-28 minutes. Allow cookies to sit on cookie sheet for a couple minutes before transferring to a cooling rack. Store in an airtight container in the refrigerator for 3-4 days.