Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan. In a mixing bowl, cream butter with granulated and brown sugars until fluffy.

Beat in egg and vanilla extract. Gradually mix in flour, cocoa powder, baking soda, and salt, alternating with buttermilk until smooth.

Pour batter into the prepared cake pan and bake for 22-25 minutes. Let cool completely.

Line a tall 9-inch round pan with acetate or aluminum foil. Slice the cooled cake into layers if desired, placing it at the bottom.

For cinnamon mousse, melt cinnamon chips and mix with canned pumpkin and half of the heavy cream. Stir in gelatin until dissolved. Fold in whipped cream and refrigerate to set.

For pumpkin-butterscotch mousse, melt butterscotch chips, then combine with the remaining heavy cream and gelatin. Fold in whipped cream and layer over set cinnamon mousse.

For salted caramel mousse, melt caramel chips, blend in caramel sauce and remaining cream, then fold in whipped cream. Layer over the second mousse.

For ganache, heat cream until simmering, pour over chocolate, let melt, stir until smooth, and pour over mousse layer.

Refrigerate for at least 4 hours or overnight. Unmold, decorate with whipped cream and candies, and serve.