Preheat your oven to 350°F (175°C). In a large mixing bowl, beat the butter with granulated sugar and brown sugar until fluffy. Add the egg and vanilla extract, mixing until combined. Gradually mix in flour, cocoa powder, and buttermilk. Pour into a lined 9-inch round pan and bake for 22-25 minutes. Allow to cool completely.

Prepare a 9-inch springform pan by lining it with acetate or foil. Place the cooled chocolate cake at the bottom, ensuring it's even.

In a saucepan, melt cinnamon chips with heavy cream over low heat until smooth. Cool slightly, stir in a pinch of salt and gelatin dissolved in water. Fold in whipped cream and spread over the cooled cake. Refrigerate for 25 minutes.

Melt butterscotch chips with heavy cream in a saucepan until smooth. Cool, then fold in pumpkin and salt. Mix in hydrated gelatin. Pour the mousse over the cinnamon layer and refrigerate until firm, about 30 minutes.

Melt salted caramel chips with heavy cream over low heat until combined. Fold in hydrated gelatin and whipped cream. Pour over the pumpkin-butterscotch mousse and refrigerate for another 30 minutes.

Heat heavy cream until just steaming, then pour over chopped semi-sweet chocolate. Whisk until smooth. Let cool slightly, then pour over the mousse layers, allowing it to cascade down the sides.

Chill the completed cake for at least 4 hours in the refrigerator. Top with whipped cream rosettes, a sprinkle of cinnamon, and festive candies. Cut with a hot knife for clean slices.