In a food processor, blend the walnuts until finely ground.
Add in the dates 2 or so at a time allowing them to incorporate into the ground walnuts until all dates have been added. Towards the end you may or may not have to pulse your processor or blender to get it to combine.
Toss in the cacao powder and pulse until the powder is evenly incorporated.
Press the dough into a 8x8 pan and refrigerate. It's ready to eat right away, but will firm up a little in the fridge making it easy to cut into brownie servings. This is quite rich, so small servings are best.