Preheat the oven to 350ºF. Spray a 9 x 13 inch glass baking dish with Pam. In a large mixing bowl, beat the eggs and sugar until the mixture is thick and pale in color. Add the pumpkin, vanilla and oil, beating well until combined. Sift the cocoa and flour over the wet ingredients then fold in gently until just combined. Add the nuts. Spread the batter into the prepared pan. Bake for 25 minutes. Cool and cut into squares.