Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan. In a mixing bowl, cream together unsalted butter, granulated sugar, and brown sugar until fluffy and light in color—about 3-5 minutes. Add in one large egg and a splash of vanilla extract, mix until fully combined. In a separate bowl, sift together all-purpose flour, unsweetened cocoa powder, baking soda, and salt, then alternate adding this dry mixture and buttermilk to the wet mixture until just 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. Allow to cool completely in the pan on a wire rack.

In a small saucepan, melt the cinnamon chips, canned pumpkin puree, heavy cream, and a pinch of salt together over low heat, stirring frequently until smooth—about 5 minutes. Allow to cool slightly, then hydrate gelatin according to package instructions before mixing it into the pumpkin mixture. Whip remaining heavy cream until stiff peaks form, then fold into the cooled pumpkin mixture. Spread this cinnamon mousse onto the cooled chocolate cake layer and refrigerate for at least 25 minutes to set.

Repeat the process for the pumpkin-butterscotch mousse layer, melting the butterscotch chips with pumpkin puree, heavy cream, and a pinch of salt, then combining with hydrated gelatin and whipped cream. Layer this on top of the chilled cinnamon mousse, smoothing evenly, then chill for another 25 minutes.

For the salted caramel mousse, melt salted caramel chips with heavy cream and a pinch of salt over low heat, then cool slightly before mixing in gelatin. Whip remaining heavy cream, fold into the cooled salted caramel mixture, and spread over the butterscotch layer. Chill the entire cake for at least 25 minutes.

To prepare the chocolate ganache, heat remaining heavy cream until it simmers, then pour over chopped semi-sweet chocolate, whisk until melted and smooth. Allow to cool slightly before pouring over set mousse layers.

Once ganache has set, remove from fridge and pipe whipped cream around the edge for decoration; sprinkle with ground cinnamon. Slice with a hot, dry knife for clean cuts, and serve.