Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine pumpkin puree, eggs, and sugar.

Sift in cocoa powder, baking powder, baking soda, and salt, then mix until just combined.

Pour the batter into a greased and floured 9-inch round cake pan.

Bake for 25-30 minutes until a toothpick inserted in the center comes out clean.

While the cake cools, prepare the chocolate mousse by melting the bittersweet chocolate over a double boiler.

Let the melted chocolate cool slightly, then in a separate bowl, whip the heavy cream until soft peaks form.

Fold the cooled chocolate into the whipped cream gently, until combined.

Once the cake is cooled completely, slice it in half to create two layers.

Spread a layer of chocolate mousse between the two cake layers, then use the remaining mousse to frost the top and sides.

Chill the cake for at least 2 hours before serving.