Preheat the oven to 350ºF and line a 9-inch square baking dish with parchment paper. (You can lightly grease the dish to help the parchment paper stay in place first.)
In a large bowl, stir together the ground chia or flax seeds and the water. This is your vegan "egg." Add in the pumpkin puree, coconut sugar, melted coconut oil, flour, cacao powder, salt, baking soda, and vinegar, and stir until a smooth batter has formed. If the dough doesn't stir easily, you can add one more tablespoon of water to help thin it out.
Fold in the dark chocolate, if using, then transfer the batter to the lined baking dish. Use a spatula to spread it evenly into the bottom of the pan-- it will be thicker than traditional brownie batter.
Bake until the center of the brownies don't jiggle, about 30 to 35 minutes. Allow the brownies to cool in the pan for at least 30 minutes before cutting into them, because they are more fragile when hot.
Slice into 16 squares and serve. Leftovers can be loosely wrapped and stored on the counter for up to 3 days, or keep them in an airtight container in the fridge for up to 2 weeks.