Preheat oven to 180°C and place shelf in the middle of the oven.
Beat butter/margarine and caster sugar together until smooth and creamy.
Add one egg and some flour and mix in, repeat until flour and eggs are gone. Add cocoa powder and mix in making sure to get lots of air into the cake mix to help it rise in the oven. Juice half an orange and add juice to mix (dont add too much orange juice or the mix will be runny and not rise properly) or add 2 teaspoons of orange flavouring.
Put cake mix into one greased 9inch cake tin and bake for 30 minutes or until cake tester comes out clean. Remove from the oven and leave to cool for a couple of minutes, then tip out onto a cooling rack.
Repeat process for second layer if you choose too.
Melt the chocolate in a microwavable bowl or jug for a about 2 minutes or place in a heat proof bowl over a pan of simmering water.
Sieve the icing suger to get out the lumps.
Beat the unsalted butter until soft and creamy (use an electric whisk if you have one as its alot easier), then add the icing sugar until light and fluffy. Slowly add the chocolate and mix together until its smooth and glossy.
Add 2 teaspoons of vanilla essence or flavouring and mix again. If you did two layers place some of the icing on one cake and sandwich together with the other. Go mad and spread the rest of the icing over the whole cake including the sides. Decorate how you want and serve and enjoy.