Preheat your oven to 350°F and grease your two Bundt pans along with the mini muffin pan. In a large bowl, combine the cake mix, pumpkin puree, egg, oil, pumpkin pie spice, and cinnamon, then beat on low before turning the mixer up to medium for about a minute and a half.
Spoon a small amount of batter into four mini muffin cups, then divide the rest evenly between your two Bundt pans. Bake the muffins for about 20 minutes and the cakes for 35 to 45 minutes total, checking with a toothpick until it comes out clean.
Let both cakes and the mini muffins cool fully before you touch the frosting step. This part takes patience, but rushing it means your buttercream will slide right off a warm cake instead of setting nicely in place.
While the cakes cool, beat the shortening on its own for a couple of minutes, then add the butter and mix until combined. Gradually add half the powdered sugar, then the milk and vanilla, followed by the rest of the sugar, beating until smooth and fluffy.
Place one cake rounded-side down and set a mini muffin in its center hole. Frost the top, then place the second cake on top rounded-side up, tucking another muffin into that center hole as well.
Color most of your frosting orange and cover the entire stacked cake with it, using an upward motion with your offset spatula to mimic a real pumpkin's ridges. Finish with a green-frosted muffin stem on top, piped vines and leaves, and a jolly black face piped on with a round tip.