Preheat oven to 350°F and lightly spray an 8-inch square baking pan with cooking spray.
Whisk together flour, cocoa powder, cinnamon, baking powder and salt in a medium bowl. Stir in walnuts.
Puree dates, eggs, egg whites, oil and vanilla In a food processor until smooth. Gradually mix wet ingredients into dry ingredients, until just combined; do not over mix. (Mixture will be very thick.)
Spread batter into prepared pan and bake for about 16 minutes or until a toothpick inserted into the center comes out clean. Let cool in pan for 30 minutes.
Run a knife around the edges to loosen brownies from pan. Invert onto a cutting board or platter. Cut into 16 pieces, wrap with plastic wrap and refrigerate overnight. Store leftovers in a covered container for up to 2 days.