1. Preheat your oven to 350°F (175°C) and prepare cake pans by greasing them and lining the bottoms with parchment paper.
2. In a large bowl, mix together the dry ingredients: all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt.
3. In a separate bowl, whisk together the wet ingredients: milk, vegetable oil, vanilla extract, and eggs until smooth.
4. Gradually add the wet ingredients to the dry ingredients, mixing until combined.
5. Slowly incorporate the hot water into the batter, mixing until the consistency is smooth.
6. Divide the batter evenly between the prepared cake pans and bake for 30-35 minutes or until a toothpick inserted into the center comes out clean.
7. Allow the cakes to cool in their pans for 10 minutes, then transfer them to a wire rack to cool completely.
8. For the mousse, whisk the egg yolks and sugar in a double boiler until creamy and slightly thickened.
9. Gradually add ½ cup of heavy whipping cream to the egg mixture, stirring constantly until the texture is custard-like.
10. Remove the mixture from heat and stir in the melted chocolate until smooth. Let it cool completely.
11. Whip the remaining heavy cream to soft peaks and gently fold it into the cooled chocolate mixture to create an airy mousse.
12. Place the first cake layer on a serving platter and spread half of the chocolate mousse evenly over it.
13. Add the second cake layer on top of the mousse and spread the remaining mousse evenly.
14. Place the final cake layer on top and prepare for frosting.
15. Make the whipped cream frosting by beating together heavy whipping cream, powdered sugar, cocoa powder, and vanilla extract until stiff peaks form.
16. Spread the whipped cream frosting evenly over the top and sides of the cake.
17. Optional: Prepare a ganache by heating heavy whipping cream until nearly boiling, then pouring it over chocolate pieces and stirring until smooth.
18. Let the ganache cool slightly and drizzle or spread it over the cake for a polished finish.
19. Chill the assembled cake in the refrigerator overnight or for several hours to allow the mousse to set fully.
20. Remove from the fridge, slice, and serve your decadent chocolate mousse cake to enjoy its rich and airy texture.