Preheat oven to 350 degrees F. Spray an 8x8 square baking dish with cooking spray.
Combine the rinsed and drained black beans, eggs, oil, cocoa powder, salt, vanilla extract, baking powder, and sweetener (of your choice - honey, sugar, etc.), in a blender; blend until smooth.
Pour the mixture into the prepared baking dish.
Bake in the preheated oven until the top is dry and the edges start to pull away from the sides of the pan, about 25- 30 minutes.
Using the whisk attachment for your mixer, beat avocado and butter until combined and no lumps remain.
Beat in vanilla and salt.
Add powdered sugar slowly and mix until combined. Add more powdered sugar to reach desired consistency.