To Make the Chocolate Cake:
1. Line a 9-inch cake pan with parchment, and spray it with non stick cooking spray. Preheat the oven to 350 F.
2. Add the butter and both sugars to the bowl of a large stand mixer fitted with a paddle attachment. Beat them together on medium-high speed, until light and fluffy, about 5 minutes. Add egg and vanilla extract, and beat well until they’re fully incorporated.
3. In a separate bowl, sift together the flour, cocoa powder, baking soda, and salt. With the mixer running on low, add a quarter of the sifted flour mixture. Add a third of the buttermilk to the mixing bowl. When that’s incorporated, continue to add the dry and wet ingredients in an alternating pattern, ending with the dry ingredients.
4. Finish mixing the cake by hand. Pour the batter into the prepared pan and smooth it into an even layer. Bake the cake for 22-25 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs attached. Cool the cake completely before using.
To Make the Mousse Layers:
1. Prepare your pan. You need a 9-inch pan with sides at least 3 inches tall, and the ability to remove the bottom. Either a cake pan with a removable bottom or a springform pan will work. Line the inner wall with a 4-inch high acetate cake collar or a strip of aluminum foil or waxed paper. Make sure the acetate or waxed paper is long enough so that it overlaps itself, and if you are using foil/waxed paper, make sure you cut the strip tall enough to extend at least an inch over the pan’s sides, since the cake is about 4 inches tall.
2. Place a cake cardboard in the bottom of the pan, then carefully place the baked cake on top of the cardboard. This is optional, but it makes it easy to transfer the cake cleanly and neatly to your serving plate.
3. Prepare the cinnamon mousse layer: combine the cinnamon chips, pumpkin puree, 1/2 cup of cream, and salt in a microwave-safe bowl. Microwave in 30-second increments, stirring every 30 seconds, until the chocolate has melted and the mixture is smooth. Pour it into a large bowl, and let it cool to room temperature, stirring occasionally.
4. While you wait for the cinnamon mixture to cool, prepare the gelatin. Whisk together the gelatin and the cold water in a small bowl, and set it aside to let the gelatin absorb the water. When the cinnamon is at room temperature, microwave the bowl of gelatin for 15 seconds, until it is melted. Whisk the melted gelatin and cinnamon mixture together, and whisk in the ground cinnamon.
5. Whip the remaining 1 cup heavy cream to firm peaks. Fold half of the whipped cream into the cinnamon, and once it’s incorporated, gently fold in the remaining whipped cream. Scrape the mousse on top of the cake, and spread it into an even layer. Refrigerate the cake for 25 minutes, until the mousse has started to set and is firm enough that a second layer can be added.
6. For the pumpkin-butterscotch mousse, combine the butterscotch chips, pumpkin puree, 1/2 cup of heavy cream, and salt in a microwave-safe bowl. Continue to make this layer the same way you made the cinnamon layer, by melting and cooling the butterscotch mixture, whisking in the melted gelatin, and folding in whipped cream. When the pumpkin-butterscotch mousse is ready, pour it over the cinnamon mousse layer. Spread it into an even layer, and refrigerate again.
7. Finally, prepare the salted caramel layer. Combine the salted caramel chips (or white chocolate chips), 1/2 cup heavy cream, salted caramel, salt, and vanilla in a microwave-safe bowl. Continue to make this layer the same way you made the previous two layers. Pour it over the pumpkin-butterscotch mousse, and refrigerate to set the mousse layers for at least 30 minutes.
8. Make a quick chocolate ganache by putting the chopped chocolate in a small bowl and heating the cream to a simmer. Pour the hot cream over the chopped chocolate and whisk gently until the chocolate melts and you have a smooth, shiny mixture. If it is very hot, let it cool slightly so it’s still warm and spreadable, but not so hot that it will melt the mousse. Pour the ganache over the top of the cake, and use a spatula to nudge it right to the edge and spread it in an even layer. Refrigerate the cake for at least 4 hours or overnight, so that the mousse layers can be very firm when slicing.
9. To finish the cake, push the bottom out from the pan, or unhinge the sides, and unwrap the acetate strip. Transfer the cake to your serving platter. Pipe rosettes of whipped cream around the edges, and sprinkle the cream with a bit of cinnamon. Top the rosettes with chocolate candies or chocolate curls, or leave them plain.
10. For the cleanest slices, cut the cake when it is well chilled. Use a large sharp knife, and wash it frequently between cuts. For the best taste and texture, allow the cake slices to sit at room temperature for 10-15 minutes before serving.