Preheat oven to 350 F. Rinse and thoroughly drain black beans.
Prepare flax egg by combining flax and water in a bowl. Set aside for 5-10 minutes.
Lightly grease a 12-slot standard size muffin pan (not mini).
Add flax eggs to blender or food processor and remaining ingredients. Retain walnuts or other toppings for later. Puree ingredients for about 3 minutes, scraping down sides as needed to form a smooth batter. If it's too thick, add a tablespoon or two of water and pulse again. It should be slightly thinner than chocolate frosting but not runny.
Evenly distribute the batter into the muffin tin and smooth the tops with a spoon.
Optional: Sprinkle with crushed walnuts, pecans or chocolate chips.
Bake for 20-26 minutes or until the tops are dry and the edges start to pull away from the sides.
Remove from oven and let cool for 30 minutes before removing from pan. They will be tender, so remove gently with a fork. The insides will be very fudgy so don't be concerned if they seem too moist.