To Make the Brownies:

1. Line a 9-by-5-inch loaf pan with parchment paper, letting a few inches of excess hang off the sides to create a sling for easy removal. 
2. Combine walnuts, dates, and ⅓ cup cocoa powder in a food processor. Pulse until the mixture is soft but crumbly. Add 2 tablespoons water and pulse again until the mixture sticks together well.
3. Transfer mixture into prepared pan and pat down firmly into an even, compact layer. Set aside. 

To Make the Frosting:

1. Place avocado, honey, ¼ cup cocoa powder, vanilla extract, cinnamon, and salt in the food processor. Pulse until smooth. 
2. Spread mixture evenly over the brownie layer. 
3. Place loaf pan in freezer for 30-60 minutes, or until firm.
4. Using the parchment sling, lift the brownies out of the loaf pan. 
5. Slice into 10 pieces and serve immediately. Store leftovers in the fridge for up to 3 days, or in the freezer for up to 2 weeks.