Preheat oven to 350 degrees. Spray a cookie sheet (with high sides-sheet pan) with non stick cooking spray and set aside.

In stand mixer or large bowl, cream butter and sugar together until light in color, 1 minute. Whip in vanilla, eggs and pumpkin until blended. Scrape sides and briefly mix again. Measure out all dry ingredients and add them in all at once. Mix on a low speed until all ingredients are moist. Scrape down the sides and mix again. Stir in sour cream. Remove bowl from mixer and stir in chopped walnuts by hand. Pour batter into prepared pan and smooth the top. Bake 15-18 minutes or until cake is cooked thoroughly. (Make sure to check it at 15!) Remove from oven and set aside to cool completely.

Using the same bowl, whip butter, vanilla, and cocoa together until well blended. Stir in powdered sugar. This mixture may look very dry, so with the mixer on, trickle in the milk. Scrape bowl and whip again until light in color, 1-2 minutes. Remove from bowl and divide into two equal portions.