Prehead oven to 350°F Coat a 10 x 15 inch baking dish with cooking spray.
Using electric mixer, in a large bowl, cream butter and sugar until light and fluffy. Add eggs and pumpkin; beat well. Add salt, pumpkin pie spice, flour, oats and baking soda; mix thoroughly. Pour batter into prepared baking dish and place walnut halves on top so there will be 1 walnut in the center of each piece of cake. Bake until a toothpick inserted in center of cake comes out clean, about 40-45 minutes.
Cool in pan and cut into 24 pieces.
Note: Batter is very thick. You will need to spread it.