Preheat oven to 350 degrees F. Grease and flour 13" x 9" baking pan.
With stand mixer, cream together vegetable shortening, and sugar. Add eggs, pumpkin, and milk.
In a separate bowl, whisk together the flour, spices, baking powder, baking soda, and salt. Add to wet ingredients and mix a few minutes until well combined.
Turn off mixer and stir in nuts by hand.
Pour cake batter into baking pan and bake for 35-40 minutes or until cake just begins to pull away from sides and a toothpick inserted in the center comes out clean.
Let cool completely. An hour at room temperature then an hour in the refrigerator ought to do it.
Frosting
With stand mixer, cream together butter, vegetable shortening, sugar, and vanilla.I like to use the whisk attachment for the frosting, but standard beaters work well, too.
Add milk mixture a little at a time.
Beat on medium-high for about seven minutes or until fluffy and smooth, scraping the sides of the bowl occasionally.The frosting will start out gritty, but the sugar will dissolve to make it nice and smooth.
Spread on cooled cake. Sprinkle with nutmeg and/or cinnamon.
Cover leftover cake with plastic wrap or tight-fitting lid and keep in refrigerator.