In a large bowl sieve the flour, cocoa powder, baking powder and bicarbonate of soda. Add the sugars and mix to combine. In a separate bowl mix together the buttermilk, oil, vanilla extract and eggs. Add the wet ingredients to the bowl of dry ingredients and start to combine. Add the hot water then mix everything together until all the ingredients are incorporated. Split the mixture between the two cake tins and bake for 30 minutes, or until cooked through. Remove from the oven and leave to sit for 10 minutes before removing the tins and cooling completely on a wire rack.
To make the buttercream beat the butter until smooth with an electric whisk, then add the icing sugar, cocoa powder, vanilla extract and milk. Continue to beat for 2-3 minutes until smooth and fluffy.
Cut each sponge in half across the centre, then layer up using ⅓ of the buttercream between each sponge. Use the rest of the buttercream to completely cover the outside of the cake evenly and neatly.