Preheat the oven to 350 degrees F (175 degrees C). Coat a 9-inch baking pan with nonstick cooking spray.

Hand whisk eggs, mashed avocado, and vanilla extract in a large bowl. Add black beans. Puree mixture with an immersion blender, until all of the black beans are mashed and batter is smooth.

Mix brown sugar and dark cocoa powder together in a separate bowl. Add to batter and blend by hand with a wooden spoon until completely mixed. Stir in chocolate chips. Pour batter into the prepared pan.

Bake in the preheated oven until a knife inserted into the center comes out clean, 25 to 30 minutes. Cool completely before cutting and serving.