Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans with butter or oil, then dust lightly with flour.

In a mixing bowl, whisk together all-purpose flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt.

In another bowl, blend vegetable oil, buttermilk, eggs, and vanilla extract until smooth.

Gradually add the wet mixture to the dry ingredients. Then, pour in boiling water and stir until just combined.

Divide the batter between the pans and bake for 25-30 minutes until a toothpick comes out clean. Cool for 10 mins before transferring to wire racks.

For the mousse, heat heavy cream in a pot until steaming, pour over chopped chocolate, and whisk until smooth.

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

Chill the mousse in the refrigerator for at least 1 hour.

For the ganache, heat heavy cream until simmering, then pour over chopped chocolate and stir until smooth.

Assemble the cake by layering mousse between the cooled cake layers and pour ganache over the top.

Refrigerate for at least 2 hours to set before serving.