Break up the chocolate and slowly heat over a bain-marie.
Add the butter and stir until silky smooth.
Add the sugar and four - sifting the flour as you go.
Add the eggs and combine until smooth.
Grate over half the rind of the orange and combine.
Line the baking trays with greased baking paper and pour in the mixture equally. If you only have one cake tin, split the mixture in half and repeat the last cooking steps with the remaining half of your mix.
Place the cake tin in the oven for 25 minutes at 180 degrees.
Add the icing sugar, butter and milk to a bowl.
Combine using a whisk until completely smooth.
Remove the cakes from the oven and allow to cool for at least 20 minutes.
Once cooled, spread the icing on top.
Add the top tier
Dust with icing sugar.
Voila you have a chocolate orange birthday cake.
I made this for a friend's son. All the kids and adults loved it.