Prepare a loaf pan by lining with parchment or plastic wrap for easy removal.
In a food processor first blend the nuts until no large pieces remain. Place the nuts in a bowl. Add the date and process until a "dough" forms - this will happen after the dates are broken up into small pieces, just continue processing.
Once the dates are fully processed, break up the dough with a knife then add the nuts, cocoa, salt and vanilla and process until thoroughly combined. The mixture may not stick together but will hold together when pinched.
Pour the mixture into the loaf pan and press down firmly.
For the frosting, process the avocados, cocoa powder, maple syrup, vanilla and salt until completely smooth and no green pieces remain, scraping down the sides of the food processor as needed. Pour the mixture over the brownies and smooth out. Cover by placing a piece of plastic wrap directly on top of the frosting so that no air can touch it as it will oxidize. Refrigerate 1-2 hours or until ready to serve.
These brownies will last up to 4-5 days covered in the fridge or can be frozen well wrapped for up to 2 months. The top of the avocado frosting may turn darker as the avocado oxidizes but this does not affect the taste