Preheat oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper.

1. In a large bowl, whisk together flour, sugar, cocoa, baking powder, baking soda, and salt. Add eggs, milk, oil, and vanilla; beat until smooth. Stir in boiling water (batter will be thin).
2. Divide batter evenly between pans and bake for 28–32 minutes, or until a toothpick inserted into the center comes out clean. Cool completely.
3. For the mousse, whip heavy cream until stiff peaks form; set aside. In another bowl, beat cream cheese, powdered sugar, pumpkin puree, pumpkin spice, and vanilla until smooth. If using gelatin, bloom in warm water and mix in. Gently fold whipped cream into the pumpkin mixture until fully combined.
4. Place one cake layer on a serving plate, spread half the mousse evenly over it, then top with the second cake layer. Spread remaining mousse over the top and sides, smoothing with a spatula. Chill for at least 2 hours to set.
5. For the ganache, heat heavy cream until just simmering, pour over chopped chocolate, let sit for 2–3 minutes, then stir until smooth. Stir in butter if desired.
6. Pour cooled ganache over the chilled cake, letting it drip down the sides. Return to the fridge until ready to serve.