Prepare the Cake:

1. Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
2. In a large bowl, sift together the flour, cocoa powder, baking powder, baking soda, sugar, and salt.
3. Add the eggs, milk, vegetable oil, and vanilla extract. Mix until smooth.
4. Gradually stir in the boiling water until well combined. The batter will be thin.
5. Pour the batter evenly into the prepared pans and bake for 30-35 minutes or until a toothpick inserted into the center comes out clean.
6. Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Prepare the Mousse:

1. Melt the dark chocolate using a double boiler or in the microwave. Allow it to cool slightly.
2. In a large bowl, whip the heavy cream, sugar, and vanilla extract until stiff peaks form.
3. Gently fold the melted chocolate into the whipped cream until well combined.

Assemble the Cake:

1. Place one cake layer on a serving plate. 
2. Spread half of the chocolate mousse over the top.
3. Place the second cake layer on top and press down gently.
4. Spread the remaining mousse over the top and sides of the cake.

Prepare the Ganache (if using):

1. Heat the heavy cream in a small saucepan until it just begins to boil. 
2. Remove from heat and pour over the chopped chocolate. 
3. Let it sit for a few minutes, then stir until smooth.
4. Allow the ganache to cool slightly, then pour over the top of the cake, letting it drip down the sides.

Refrigerate and Serve:

1. Refrigerate the cake for at least 2-3 hours to allow the mousse to set.
2. Slice and serve.