For the buttercream: Place the butter and shortening in the bowl of a stand mixer fitted with the paddle attachment. Mix on medium speed until well incorporated.
Sift the powdered sugar if needed. Add the sugar to the bowl 1 cup at a time. Mix on low speed until incorporated, and then increase the speed to medium-high and allow to double in volume and lighten in color. Add the vanilla and salt. Stream in a little oil at a time until the buttercream is smooth and light. Use immediately or store at room temperature up to 3 days or refrigerated for a week.
For the cake: Separate the buttercream into 3 bowls and color one yellow, one orange and one dark orange. Place the colors into piping bags and cut the tip.
Remove your cake from the freezer and place on a turntable cake stand. Ice the cake with a layer of the yellow icing. If the icing stiffens from the chill of the cake, then you are ready for the next layer of icing. If the icing is still soft, place the cake in the freezer and allow to set, about 1 hour (or overnight in the refrigerator).
Once the yellow layer is hard, ice the cake in light orange and follow the same steps as the first layer.
Ice the cake in the final layer of dark orange and place the cake in the refrigerator for up to 2 days until you are ready to carve.
Once the cake is chilled, remove from the refrigerator and start carving. You can use a stencil of a cake found online or in a pumpkin carving book or from your imagination. Have fun and be creative. Involve your kids or carve with friends.