Preheat oven to 350°F. Butter and flour four 6-inch stainless steel mixing bowls (2 1/2 cup capacity).
In a stand mixer, fitted with the paddle attachment, combine the oil, sugar, brown sugar, and eggs. Once they are well combined add the pumpkin puree, vanilla and sour cream. In a separate bowl, whisk together the flour, baking soda, salt and spices. Add the dry ingredients to the pumpkin mixture and mix until it all comes together in a smooth batter.
Divide the batter in the four bowl and place them on a baking sheet. Bake for about 40 to 50 minutes (this may depend on the shape of your bowls).
Turn them out onto a sheet and allow them to cool completely. They can be made ahead and refrigerated for up to 2 days or frozen for 2 weeks.
Make the buttercream icing (follow these directions). Separate out all but a 1/2 cup of the buttercream. Use the food coloring to create the orange. Save the remaining buttercream to use for writing on the cake. Spread a thin layer of buttercream over the flat sides of the cakes. Sandwich the two pieces together to make a sphere.
Spread a thick layer of icing over the entire cake, just leaving the very bottom free of icing. Use a spatula to create the stripes along the sides of the pumpkin, then flatten out the top just slightly.
Refrigerate the pumpkin cake to set the buttercream. Repeat with the other two half rounds to create a second cake.
While the cake is chilling, color the marzipan with green food coloring. Use gloves to keep your hands clean.
Sculpt stems and the curly tendrils from the green marzipan. Allow them to sit out at room temperature for a few hours or overnight so they will hold their shape. Set the chilled cake on a stand and decorate with a stem and curly tendrils.