Preheat the oven to 180°C / 160°C / 350°F / Gas Mark 4. Line 2 x 20cm (8-inch) round cake tins with baking/parchment paper and set aside.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, bicarbonate of soda, baking powder and salt until thoroughly combined.
In a medium-sized bowl or jug, whisk together the buttermilk, oil, eggs and vanilla extract.
Now add the wet ingredients along with the hot coffee and carefully whisk until just mixed and no lumps of flour remain.
Distribute the cake batter evenly between the two cake tins and bake for 35-40 minutes or until a cake tester comes out clean without any uncooked batter attached. Please Note: The original recipe instructs you to bake the cake for 35–40 minutes, mine took nearer to 60 minutes to fully cook through. All ovens are different, so you may need to check the cake at 40 minutes and keep baking until the cake tester comes out clean.
Once the cakes are baked all the way through, allow them to cool in the tins for 30 minutes, then remove from the cake tins and leave to cool on a wire rack completely.
To make the buttercream, start by creaming the butter, icing sugar and vanilla extract until light and fluffy. This should take about 5 minutes by hand or by using an electric mixer. Now add the cocoa powder and mix again until just combined. Finally, add the double cream and mix again until combined.
Take the first cake layer and place directly on your chosen serving dish/plate. I used a cake board and spread a tiny bit of buttercream on the base to secure the first cake layer. Take half the buttercream and spread it out evenly, then place the second cake layer on top and spread the remaining buttercream on top. I like to pipe a few swirls of the buttercream on top of my cake.
Decorate the cake further with your favourite chocolates or sprinkles. The cake will keep stored, covered at room temperature for up to 3 days.