Preheat oven to 350°F (175°C). Grease and line a round cake pan.

1. In a large bowl, mix flour, cocoa powder, baking powder, baking soda, and sugar.
2. Add eggs, milk, oil, and vanilla extract. Mix until smooth. Stir in boiling water (batter will be thin).
3. Pour into the prepared pan and bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean. Let cool completely.
4. For the mousse, melt the chocolate and allow it to cool slightly.
5. Whip the heavy cream until stiff peaks form.
6. Gently fold the melted chocolate into the whipped cream until fully combined.
7. If using gelatin, dissolve it in warm water and mix it into the mousse for added stability.
8. Place the cooled cake in a springform pan. Pour mousse over the top and smooth the surface.
9. Refrigerate for at least 4 hours or overnight until set.
10. Remove from pan, garnish as desired, slice, and serve chilled.