Preheat your oven to 350°F (175°C) and grease and flour two 9-inch round cake pans.

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

In a separate bowl, whisk the vegetable oil, buttermilk, eggs, and vanilla extract until smooth.

Pour the wet mixture into dry ingredients and mix until just combined, then slowly add the boiling water until the batter is smooth.

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

Let the cakes cool in the pans for about 10 minutes, then transfer them to cooling racks to cool completely.

To prepare the mousse, heat 1/2 cup of heavy cream until steaming and pour it over chopped chocolate; whisk until smooth.

Whip the remaining heavy cream to soft peaks and fold it into the chocolate mixture, then chill for at least 1 hour.

For the ganache, heat 1 cup of heavy cream until simmering and pour over chopped chocolate, stirring until smooth.

After the cakes are cool, place one layer on a platter, spread mousse on top, add the second layer, cover with remaining mousse, and pour ganache over.