Preheat oven to 350°F.
Mix the ground flaxseed with the water in a small bowl and let sit for about 5 minutes until it forms a gel.
In a high-powered blender or food processor, blend the dates with the soy milk until smooth.
Add the black beans and blend again.
Add the flax egg, oats, cocoa powder, vanilla, baking powder, sea salt, and optional chia seeds.
Blend until completely smooth and creamy, scraping down the sides as needed.
Stir in the optional chocolate chips or chopped nuts.
Pour into a parchment-lined 8x8-inch baking dish (or divide into a muffin tin).
Bake for about 25 minutes, or until the center is set and a toothpick comes out mostly clean.
Allow the brownies to cool completely before slicing. This helps them firm up into that perfectly fudgy texture.