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.