Preheat oven to 350 degrees. Line a square baking pan with parchment paper.
Prepare brownie mix according to package directions and bake for about 30-40 minutes or until a toothpick inserted two inches from the edge of the pan comes out clean. Let brownies cool fully.
Slice the avocado and place in a food processor along with the cocoa powder and maple syrup. Process on high, stopping to scrape down the sides occasionally until very smooth and combined.
Taste the frosting and add more cocoa powder or maple syrup if needed. The frosting will be thick. Spread over completely cooled brownies. Store leftover brownies in the refrigerator.