Preheat the oven to 170°C (340°F) .

Line a 20 cm (8 inch) square cake tin.

In a bowl, combine the flour and sugar with a wooden spoon.

In a small saucepan combine the water, cocoa, oil and butter. Heat until combined and the butter is melted.

Add the warm mixture to the dry ingredients and stir until combined.

In a separate bowl, mix together the eggs, buttermilk, vanilla and bicarbonate of soda. Add to the flour and butter mixture in the large bowl and mix well until combined.

Pour into the prepared tin and bake for 25 minutes or until cooked through and a skewer in the middle comes out clean.

While the cake is in the oven, prepare the icing by melting the butter and mixing with the buttermilk and cocoa. Stir well to combine. Add the icing sugar, mixing well until the icing is super smooth.

As soon as the cake is removed from the oven, pour the icing over the top.

If you wish, decorate with chopped honeycomb or maltesers.