Make the cake: Preheat the oven to 350˚F (180C˚). Butter (20*320cm) pan (or similar size) and line the bottom with a parchment paper, grease the parchment with butter, sprinkle 1-2 tablespoons of cocoa powder and shake to get rid of excess cocoa. Set aside.
To a large bowl add flour, cocoa powder, sugar, baking powder, baking soda and salt. Stir well and set aside.
In a separate bowl mix buttermilk, coffee (or water), eggs, oil and vanilla extract. Mix until well combined.
Add the wet mixture to the dry, whisk until incorporated.
Pour the batter into prepared pans.
Bake for 30-35 minutes or until a toothpick comes out clean. Let cool completely before frosting.
Cut the top of the cakes to make the cake flat and esthetic. Place one the back to the pan.
Make the cream: in a large bowl whip the cream cheese with powdered sugar and vanilla extract. When smooth pour in heavy cream and whip until stiff peak form.
Spread the cream over the first layer of cake, top with the second cake. Refrigerate for at least 3 hours.
Make the topping: chop the chocolate and place in a heatproof bowl. In a small saucepan heat heavy cream. Pour hot cream over the chocolate, allow to sit for 2 minutes, then stir until melted and smooth. Reserve 2 tablespoons of the ganache for later use.
Pour chocolate ganache over the cake and allow to set. Decorate the cake with reserved chocolate ganache.
Notes:Why buttermilk?The use of buttermilk in cakes and cupcakes helps the cake to get a moist and airy texture, in most of my recipes I use buttermilk and indeed you can feel the difference.What can we use instead of buttermilk?If you cannot find buttermilk in your country, you can make a home buttermilk: stir 1 cup (240ml) of whole milk with 1 tablespoon of vinegar or lemon juice, let stand for 10 minutes before use. If you still prefer not to use buttermilk you can use whole milk instead.