Preheat the oven to 375°F. Line a small baking sheet with parchment paper.
In a small bowl, combine the ground flaxseed and water. Stir vigorously and then set aside for 5 minutes, allowing it to thicken to a gel-like consistency.
In a medium bowl, combine the almond meal, cocoa powder, coffee (if using), salt, and baking soda. Stir to combine.
To the flaxseed gel, add the maple syrup, peanut butter, avocado oil, and vanilla. Whisk until thoroughly combined. Add this wet mixture to the almond meal mixture. Using a spatula, stir until the thick and slightly wet dough is combined and there are no dry spots.
Add the chocolate chunks to the dough and fold them in until evenly distributed throughout.
Transfer the cookie dough to the baking sheet and press it out into an even circle that is uniformly ½-inch thick. The overall circumference should be 5 inches or so. Press more chopped chocolate into the top of the cookie if you like.
Bake the cookie until small cracks appear along the edges and top, and the colour of the cookie has lightened up a bit, about 10 minutes. Let the cookie cool completely (at least 30 minutes) on the baking sheet before enjoying.