Preheat your oven to 350°F (175°C). In a mixing bowl, whisk together all-purpose flour, cocoa powder, baking powder, baking soda, and salt. In another bowl, cream the softened unsalted butter and granulated sugar until light and fluffy. Beat in the eggs and vanilla extract, then gradually add the dry ingredients to the wet mixture. Pour the batter into a greased 9-inch round cake pan and bake for 25-30 minutes, or until a toothpick inserted comes out clean. Let the cake cool completely on a wire rack.

In a mixing bowl, whip the heavy cream until soft peaks form, then set it aside. In another bowl, combine pumpkin puree, granulated sugar, cinnamon, and nutmeg. Dissolve unflavored gelatin in water according to the package instructions, then stir it into the pumpkin mixture. Gently fold in the whipped cream until fully combined.

Once the chocolate cake has cooled, carefully slice it in half horizontally to create two layers. Place the bottom half on a serving plate and spread a generous layer of pumpkin mousse over it. Place the second layer of chocolate cake on top, pressing down gently, and spread more pumpkin mousse over the top and sides.

In a small saucepan, heat heavy cream until just boiling, then pour it over semi-sweet chocolate chips in a bowl. Let it sit for a few minutes, then stir until smooth to create the ganache. Once the mousse has set in the refrigerator for at least 2 hours, pour the ganache over the chilled mousse cake, allowing it to drip down the sides.

Once the ganache is firm, decorate your Pumpkin Chocolate Mousse Cake with dollops of whipped cream around the edges and sprinkle with chocolate shavings or pumpkin candies. Slice with a serrated knife and serve chilled.