Pre-heat the oven to 160C (320F) fan-forced and grease a 20x25cm cake pan then flour it well. You can also use two 20cm round tins and sandwich the finished cakes together.
Place all the dry ingredients (flour, sugar, cocoa, baking soda, baking powder and salt) in the bowl of a stand mixer.
Add all the wet ingredients (eggs, coffee, buttermilk, oil and vanilla).
Place the bowl on the mixer and beat it all together for about two minutes. This can be done using a hand-held mixer too.
Pour the batter into the prepared tin or tins.
Bake the cake for 35 - 40 minutes but check it after 35 minutes. I used a very deep tin so mine took about 50 minutes to cook. The cake is cooked when a tester, inserted in the middle of the cake comes out clean.
Leave the cake in the tin to rest for about 10 minutes before transferring to a wire rack to cool completely.
While the cake is cooling, make the icing by beating all the ingredients together until light and fluffy.
Spread the cooled cake with the icing and serve it up.