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.
for the caramel frosting-
Whip butter, vanilla, and caramel syrup together until well blended. Whip powdered sugar into mixture until incorporated. Scrape the sides of the bowl and whip again until light in color, 1-2 minutes. Remove from bowl and divide into two equal portions. Set aside.
for the chocolate frosting-
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.
Cut cooled cake into 4 equal strips width wise. Carefully remove 1 strip of cake and place onto serving platter. Top with half of chocolate frosting, being sure to have thicker frosting around the edges and thinner frosting in the middle. Repeat this process alternating the chocolate and caramel frostings, finishing with caramel. Top cake with chopped walnuts and toffee bits. Serve.