Using a food processor, process the nuts until they are the consistency of nut flour.
Add the cacao and sea salt to the walnut flour and pulse to blend well.
Optionally, add the vanilla extract or powder and pulse again to blend well.
Add one date at a time to the mixture through the feed tube on the processor. Don't add too many dates—just enough so the batter resembles cake crumbs, but when pressed between your fingers, it sticks together. The more dates you add, the better it will stick together, but the denser the brownie will be. I prefer them lighter, so I use the smallest amount of dates necessary to get the batter sticking together.
Empty the batter into a dish (I use a square 8 by 8 inch dish), and press down evenly.
Place in the refrigerator for about 10 minutes, and then slice them into cubes with a knife. They are easier to slice when a bit cold. Store cold, or at room temperature.