Step 1: Make the Chocolate Cake Base

1. Preheat your oven to 350°F (175°C). Grease a 9-inch springform pan and line the bottom with parchment paper.

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

3. In another bowl, whisk the sugar, vegetable oil, eggs, and vanilla extract until smooth.

4. Add the buttermilk to the wet ingredients, mixing well. Then, gradually add the dry ingredients, stirring until just combined.

5. Finally, pour in the hot water (or brewed coffee) and mix until the batter is smooth and slightly runny.

6. Pour the batter into the prepared pan and bake for 22-25 minutes, or until a toothpick inserted in the center comes out clean.

7. Let the cake cool in the pan for 10 minutes, then remove it from the pan and transfer to a wire rack to cool completely.

Step 2: Prepare the Chocolate Mousse Layer

8. In a heatproof bowl, melt the chocolate chips and butter together using a double boiler or in short bursts in the microwave, stirring until smooth.

9. Let the melted chocolate cool slightly, then stir in vanilla extract and salt.

10. In a separate bowl, whip the cold heavy cream using an electric mixer until soft peaks form.

11. Gently fold the whipped cream into the melted chocolate mixture, being careful not to deflate the mousse.

12. Place the cooled cake base back into the springform pan. Spread the mousse evenly over the cake, smoothing the top with a spatula.

13. Refrigerate for at least 4 hours, or until the mousse is fully set.

Step 3: Make the Ganache Topping

14. Heat the heavy whipping cream in a saucepan over medium heat until it starts to simmer. Remove from heat and pour over the chocolate chips in a heatproof bowl.

15. Let sit for a minute, then stir until smooth and glossy. Add the honey or corn syrup for extra shine.

16. Let the ganache cool slightly before pouring it over the chilled mousse layer. Spread evenly with an offset spatula.

Step 4: Chill and Serve

17. Return the cake to the fridge for another 30 minutes to allow the ganache to set.

18. Once ready, carefully remove the cake from the springform pan. Slice and serve chilled!