Preheat your oven to 350°F (175°C) and prepare two 9-inch round cake pans by greasing and flouring them.

In a large mixing bowl, whisk 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.

Gradually pour the wet mixture into the dry ingredients, stirring gently. Slowly add boiling water to the batter.

Divide the batter between the prepared pans and bake for 25 to 30 minutes.

Cool the cakes in the pans for 10 minutes, then invert onto wire racks to cool completely.

For the mousse, heat 1/2 cup heavy cream until simmering, pour over chopped semisweet chocolate, and whisk until smooth.

Whip the remaining heavy cream with sugar and vanilla extract until soft peaks form, then fold into the chocolate mixture.

Cover the mousse and chill in the refrigerator for at least 1 hour.

To make the ganache, heat 1 cup of heavy cream until simmering, pour over chopped semisweet chocolate and stir until smooth.

Assemble the cake by placing one layer on a platter, spreading half the mousse over it, topping with the second layer, and spreading the remaining mousse.

Pour ganache over the assembled cake and chill in the refrigerator for at least 2 hours before serving.