Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans with butter or oil; dust with flour.

In a large bowl, sift together the flour, sugar, cocoa powder, baking soda, baking powder, and salt until well combined.

In another bowl, whisk together the vegetable oil, buttermilk, eggs, and vanilla extract until smooth.

Gradually add the boiling water to the wet ingredients while stirring. The batter will be thin.

Divide the cake batter evenly between the prepared pans and bake for 25-30 minutes until a toothpick comes out clean.

Heat ½ cup of heavy cream over low heat until it simmers, then pour it over the chopped semisweet chocolate and whisk until smooth; cool slightly.

Whip the remaining 1½ cups of heavy cream to soft peaks and gently fold in the cooled chocolate mixture until just blended.

Heat another ½ cup of heavy cream until it simmers, pour it over 6 ounces of chopped semisweet chocolate, stirring until smooth.

Once the cake layers are cool, place one layer on a plate, spread half of the mousse, and top with the second layer; frost with remaining mousse.

Pour the cooled ganache over the cake and allow it to set for about 30 minutes at room temperature, then refrigerate for at least 2 hours.