Preheat oven to 350°F (175°C). Grease and line the bottom of a springform pan.

1. Prepare the chocolate cake: In a bowl, whisk together flour, cocoa, baking powder, baking soda, salt, and sugar. Add eggs, milk, oil, and 1 tsp vanilla; mix until smooth. Stir in boiling water until well blended. Pour into prepared pan and bake for 25–30 minutes or until a toothpick comes out clean. Let cool completely.
2. Prepare the pumpkin mousse: In a large bowl, beat cream cheese until smooth. Add pumpkin puree, brown sugar, cinnamon, nutmeg, ginger, and 1 tsp vanilla; mix until combined. In a separate bowl, whip 1 cup heavy cream to stiff peaks and gently fold into pumpkin mixture.
3. Prepare the chocolate mousse: Melt chopped chocolate in a heatproof bowl over simmering water or in the microwave in short bursts. Cool slightly. In another bowl, whip remaining 1 cup heavy cream, powdered sugar, and 1 tsp vanilla to stiff peaks. Fold melted chocolate into whipped cream until smooth.
4. Assemble the cake: Spread pumpkin mousse evenly over cooled chocolate cake layer. Top with chocolate mousse and smooth surface.
5. Refrigerate at least 4 hours or until set. Garnish with chocolate shavings or cocoa powder before serving.