Add walnuts, ground almonds and cacao to a food processor and blend until it resembles fine crumbs.
Now add in dates and syrup and blend until everything starts to come together. It shouldn't be sticky, but should press together easily in between your fingers. Press the base into a lined baking tin, smoothing everything out evenly. Set aside in the fridge to chill while you prepare the frosting.
In a small bowl, add cocoa, maple syrup and almond butter and mix well with a spoon. It should be thick but spreadable, so add soy milk a tbsp at a time if you need to thin it out. Spread on top of the brownie base and pop back in the fridge to chill for 30 mins.
Notes
If you don't have cacao (unrefined cocoa powder), regular cocoa powder will work just as well. You can also try subbing carob instead of cacao - I tried this on one of the batches and it was delicious!
If you don't have maple syrup you can sub with agave or date syrup.
Dates super dry? Simply soak them in hot water for 15 minutes before using.
Any nut butter can be subbed for the almond butter, just make sure it's smooth.