Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan. In a large mixing bowl, cream together room-temperature unsalted butter, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes. Add in a room-temperature egg and vanilla extract, mixing well. Gradually incorporate the dry ingredients along with buttermilk until combined. Pour the batter into the prepared pan and bake for 22-25 minutes, or until a toothpick inserted in the center comes out clean. Cool the cake completely on a wire rack.

In a medium saucepan over low heat, melt the cinnamon chips with 1 cup of heavy cream until smooth. Remove from heat and stir in the pumpkin puree. In a small bowl, dissolve the unflavored powdered gelatin in cold water per the packet instructions, then mix it into the pumpkin mixture. Allow to cool slightly, then gently fold in whipped cream until no streaks remain. Once the chocolate cake has cooled, spread the cinnamon mousse layer evenly on top.

Melt butterscotch chips with 1 cup of heavy cream in a saucepan over low heat, stirring until smooth. Remove from heat and let cool. Meanwhile, dissolve gelatin in cold water as directed. Once cooled, mix the gelatin into the butterscotch mixture and fold in whipped cream until well combined. Carefully layer this mixture over the cinnamon mousse, smoothing it out with a spatula.

To make the salted caramel mousse, melt the salted caramel chips with 1 cup of cream in a saucepan over low heat, stirring until silky. Remove from heat and allow to cool, then mix in the thick salted caramel sauce. Prepare the gelatin by dissolving it in cold water per instructions. Incorporate the gelatin into the cooled caramel mixture, then carefully fold in whipped cream to lighten. Spread this layer on top of the butterscotch mousse, smoothing it out.

For the ganache, heat the remaining 1 cup of heavy cream in a saucepan until it simmers. Remove from heat and pour it over the chopped semi-sweet chocolate in a heatproof bowl. Let it sit for a couple of minutes, then stir until smooth and glossy. Allow the ganache to cool slightly before pouring it over the layered mousses, ensuring an even coating.

Cover the Pumpkin Chocolate Mousse Cake with plastic wrap and refrigerate it for at least 4 hours, or overnight for the best results. This chilling time helps the layers set properly and meld the flavors together for a deliciously rich dessert.

Once set, remove the cake from the fridge. Decorate the top with dollops of whipped cream and sprinkle with chocolate candies for an elegant touch. Use a hot, dry knife to slice cleanly through the layers, revealing the beautiful Pumpkin Chocolate Mousse Cake inside as you serve it to your delighted guests.