Preheat the oven to 350°F. Line two 6-inch cake pans with parchment paper.
To make the batter: In the bowl of a stand mixer fitted with the paddle attachment, add the butter, cane sugar, pumpkin spice, and vanilla extract. Mix on low until combined and there are no butter chunks.
Add the eggs, milk, pumpkin, and sour cream to the butter mixture and mix on low until combined.
Add the cake flour, all purpose flour, baking powder, and salt to the butter mixture and mix on low until combined into a smooth batter. Be sure to scrape down the sides of the bowl to ensure the batter is mixed completely.
Transfer the batter to the prepared cake pans, each should weigh about 454 grams each.
Bake for 35 to 40 minutes or until a toothpick inserted in the center comes out clean. Allow the cakes to cool completely before assembling.
To make the frosting: Add the heavy whipping cream, powdered sugar, and vanilla extract to a stand mixer fitted with the whisk attachment. You could also use a hand mixer if you prefer, but I think it’s much faster in a stand mixer. Whisk, starting on low, to avoid splattering, then slowly increase the speed until you are at full speed. Whisk until stiff peaks form.
Transfer the whipping cream frosting to a piping bag with Ateco tip #827.
To assemble the cake: Place the first cake layer (bottom side up) on a cake stand or plate (whatever you plan to serve it on). You don’t really need a turntable for this, but you can put the plate on it for ease if you prefer. Pipe a shell boarder by piping a swooping motion up and then towards you. You could think of it as writing a cursive “r” that is lazy and rounded or just a hump like on a country road. (Watch the video!). Pipe swoops around the entire boarder of the first cake layer. Then pipe them in the center too (center is not important as you won’t see it, so you could also just do a big dollop of frosting!).
Place the second cake layer on top (bottom side up). Pipe the same shell boarder (or swoops) around the edge of the cake. Then pipe a few more to fill the center.
Top the cake with two cinnamon sticks (optional).
Store in an airtight container in the fridge for up to 3 days.