Preheat the oven to 350 degrees F (180 degrees C) and line an 8-by-8-inch baking pan with parchment.
Make the flax "eggs": Stir 2 tablespoons ground flax with 5 tablespoons water in a small bowl. Let sit for 15 minutes. (See Cook's Note.)
Meanwhile, combine the almond flour, sugar, cocoa powder, baking soda and salt in a large bowl; set the dry mixture aside.
In a medium, microwaveable bowl, melt 3 ounces (about 1/2 cup) of the chopped chocolate. Add the almond butter and stir to combine. Let cool slightly and stir in the coconut oil, then the maple syrup, vanilla and flax "eggs" and whisk until evenly combined.
Whisk the dry mixture into the wet until just combined (be careful not to over mix). Then fold in the remaining 2 ounces (about 1/3 cup) chopped chocolate.
Transfer the batter to the prepared baking pan and bake until just set in the center, 20 to 25 minutes.
Let cool completely to room temperature, then cut into squares. Serve or cover and refrigerate for up to 4 days or freeze and thaw for another time.
If you wish to make a paleo version with eggs, use 1 large egg plus 1 large egg yolk at room temperature instead of the flax "eggs."