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

In a large bowl, combine flour, sugar, cocoa powder, baking soda, baking powder, and salt. Set aside.

In another bowl, whisk together vegetable oil, buttermilk, eggs, and vanilla extract. Gradually add the wet ingredients to the dry ingredients, mixing until just combined.

Slowly add the boiling water to the batter, mixing gently until smooth. The batter will be thin.

Divide the batter evenly between the prepared cake pans.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Cool the cakes in the pans for 10 minutes before transferring to a wire rack to cool completely.

To prepare the chocolate mousse, heat 1/2 cup of heavy cream until steaming. Pour over the chopped chocolate and let sit for 5 minutes. Whisk until smooth.

In a separate bowl, whip the remaining 1 1/2 cups of heavy cream with sugar and vanilla until soft peaks form.

Gradually fold in the chocolate mixture until well combined. Refrigerate for at least 1 hour or until set.

For the ganache, heat the heavy cream until it simmers. Pour over the chopped chocolate and let sit for 5 minutes. Stir until smooth and glossy.

Once the cakes are cool, place one layer on a serving plate. Spread a layer of chocolate mousse over the cake.

Place the second cake layer on top and cover the entire cake with mousse.

Pour the ganache over the cake, allowing it to drip down the sides. Use a spatula to smooth the top if needed.

Chill in the refrigerator for at least 2 hours before serving.