Preheat the oven to 350 degrees F.
For the cake: Butter and flour the stainless steel bowls. In a stand mixer fitted with the paddle attachment, combine the oil, granulated sugar, brown sugar and eggs. Once they are well combined add the pumpkin puree, sour cream and vanilla. In a separate bowl, whisk together the flour, baking soda, nutmeg, ginger, allspice and salt. Add the dry ingredients to the pumpkin mixture and mix until it all comes together in a smooth batter.
Divide the batter among the four prepared bowls and place them on baking sheets. Bake until a toothpick comes out clean, about 40 to 50 minutes (this may depend on the shape of your bowls). It is okay if they don't bake completely flat, chances are they won't.
Turn the cakes out onto a baking sheet and allow them to cool completely. They can be made ahead and refrigerated for up to 2 days or frozen for 2 weeks.
In the bowl of your stand mixer, combine the sugar and egg whites. The mixture will be very thick and grainy. Put the bowl over a double boiler and stir with a rubber spatula until the sugar is completely melted. This can take several minutes. You want to brush the sides down with the spatula to make sure all the sugar is melted and no grains are clinging to the sides. Feel the egg mixture between your fingers to check for graininess. Once it is completely smooth and has no grains left, put it on your stand mixer and beat with the whip attachment on medium high speed. Beat it until it is light, fluffy, glossy and the bowl feels just about room temperature. (If the whites are not cooled off sufficiently it will melt the butter when you add it.)
Once the egg whites are whipped and cooled, add the butter, 2 tablespoons at a time, on medium speed. WARNING: After you have added about half of the butter, the mixture may look curdled and runny. This is normal and you should continue adding the rest of the butter. Once you have finished adding the butter and it has mixed on medium speed for about a minute, the buttercream should be creamy and glossy-looking again. Add the vanilla and salt. Use food coloring to achieve the color orange you like.
Assemble the cake: Spread a thin layer of buttercream over the flat side of each cake. Sandwich 2 of the cakes together to make a sphere.
Spread a thick layer of icing over the entire cake, leaving just the very bottom free of icing. Use a spatula to create the stripes along the sides of the pumpkin, and then flatten out the top just slightly.
Refrigerate the pumpkin cake to set the buttercream, at least 1 hour. Repeat with the other two half rounds to create a second cake.
While the cakes are 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 cakes on stands and decorate with the stems and curly tendrils. Serve at room temperature. It is ok to leave the decorated cake at room temperature for up to 24 hours.
four 6-inch stainless steel mixing bowls (2 1/2-cup capacity each)