Preheat the oven to 160°C fan / 180°C
With an electric whisk, beat together the caster sugar and butter until light and pale.
Add in the eggs and continue to beat the mixture.
Add in the vanilla extract, orange juice and zest followed by sieving in the cocoa powder and self-raising flour. Fold in the flour until the mixture is combined.
Spoon the mixture into the cupcake cases and place in the oven for 15 minutes or until a skewer comes out clean from the cupcakes.
Place the cupcakes on a wired rack to cool fully before icing.
To make the buttercream, beat together the butter and icing sugar.
Add the zest of the orange and the juice to loosen the mixture.
Place the mixture in a piping bag with your choosen nozzle and pipe on top of the cupcakes. Add the orange segment to the top.