Combine dates, walnuts, 6 Tbsp. cacao powder, 1 1/2 tsp. vanilla, 2 tsp. water and salt in a food processor. Process until completely smooth.
Lightly grease an 8-inch square baking dish, or line dish with parchment or waxed paper. Transfer dough to dish. Press dough very firmly into dish with your hands until dough is evenly distributed.