Lightly grease an 8x8 baking pan or a 12-slot standard size muffin pan (not mini // as original recipe is written // adjust if altering batch size).
Prepare flax eggs by combining flaxseed meal and water in the bowl of the food processor. Pulse a couple times and then let rest for a few minutes.
Add remaining ingredients through baking soda and puree – about 3 minutes – scraping down sides as needed. You want it pretty smooth.
If the batter appears too thick, add a Tbsp or two of water and pulse again. It should be slightly less thick than chocolate frosting but nowhere close to runny. Add almost all of the chocolate chips and stir.
Evenly distribute the batter into the 8x8 or muffin tins and smooth the tops with a spoon or your finger.
Measure out peanut butter in a mixing cup and add in powdered sugar - mix to combine.
To the brownie pan, add dots of the peanut butter powdered sugar mixture and swirl with a toothpick or knife. Sprinkle top with a few more chocolate chips or shaved chocolate.
Bake for 20-26 minutes if using muffin tins or 35-45 minutes if using an 8x8 dish. You want the top dry and the edges to start to pull away from the sides. It shouldn't be jiggly when shaken.
Remove from oven and let cool for 30 minutes in the pan before serving. They will be tender, so remove gently with a spatula or fork. If they're too tender, let set in the freezer to stiffen so they're easier to lift out. The insides are meant to be very fudgy, so don’t be concerned if they seem too moist – that’s the point. Plus, they’re vegan, so it doesn’t really matter.
Store in an airtight container for up to a few days. Refrigerate to keep longer.