Preheat your oven to 350°F (175°C) and prepare an 8-inch round or springform pan by greasing and lining it with parchment paper

Whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt in a medium bowl

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

Gradually mix the wet ingredients into the dry, then stir in the hot water carefully until the batter is smooth and thin

Pour the batter into the pan and bake for 30–35 minutes, or until a toothpick comes out clean from the center

Let the cake cool for 10 minutes in the pan, then transfer to a wire rack to cool completely

To make the mousse, place the chopped chocolate in a heatproof bowl

Heat the heavy cream and sugar in a saucepan until it just begins to simmer, then pour it over the chocolate

Let it sit for 2 minutes, then stir until smooth and melted; stir in vanilla and let cool to room temperature

Whip the remaining heavy cream to stiff peaks, then gently fold it into the chocolate mixture until fluffy and well combined

Spread the mousse over the cooled cake using a spatula and refrigerate for 2–3 hours or overnight until set

For the ganache, heat the cream until simmering and pour it over the chopped chocolate; let sit, then stir until smooth

Let the ganache cool slightly, then drizzle over the mousse topping

Garnish with whipped cream and chocolate curls if desired before serving