Whisk flour, cocoa powder, cinnamon, baking powder and salt in a medium bowl. Stir in walnuts and set aside. In a food processor, combine dates, eggs, egg whites, oil and vanilla. Purée until combined and smooth.
Gradually mix wet ingredients into dry ingredients, until just combined; do not over-mix. (Mixture will be very thick.)
Lightly spray sides of 8 x 8-inch (20 cm) baking pan with cooking spray. Pour batter into pan and place into oven. Bake for about 16 minutes or until a toothpick inserted into the center comes out clean.
Remove pan from oven and allow brownies to cool to room temperature, about 30 minutes.
Loosen brownie and turn slab out onto a cutting board or platter. Cut into 16 pieces, wrap with plastic wrap and refrigerate overnight.
Leftovers may be kept refrigerated in a re-sealable container for up to 2 days.