Preheat oven to 350 degrees. Lightly grease a 12-slot standard size muffin pan. Make sure you've rinsed and thoroughly drained your black beans. Prepare flax egg by combining flax and water in the bowl of the food processor. Pulse a couple times and then let rest for a few minutes. Add remaining ingredients (besides walnuts or other toppings) 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. Evenly distribute the batter into the muffin tin and smooth the tops with a spoon. Optional: Sprinkle with crushed walnuts or pecans.