Sift together the dry ingredients and whisk them together.
In a separate bowl whisk together buttermilk, oil, eggs and vanilla.
Slowly whisk this into the dry mix until you have a smooth, lump free batter. Stir in the coffee.
Pour into a large lined baking tray and bake for around an hour. Don't open the oven door!
Meanwhile, heat a small amount of water in a saucepan. Set a heatproof glass bowl above it and add the eggs, sugar and salt, Stir together until they reach 70°C.
Remove from the heat and whisk vigorously until they form stiff peaks. Keep whisking until they are below 30°C.
Add the butter, a cube at a time. At one point the mixture will look like it's curdled, but keep going and it'll smooth out again.
When the cake is cool, cut it evenly in half and smooth the meringue buttercream between the two halves. Pop the cake in the fridge.
Heat the cream until it starts to steam and feels hot to the touch, then pour it over the chocolate and gently fold it all together to make a smooth ganache. Allow it to cool and thicken before pouring over the cake.
Pop in the fridge to let the ganache set and serve. Yum!