Preheat the oven to 170˚C fan / 190˚C / 340˚F convention / 375˚F
Line 6 cupcake/muffin tins with 6 cupcake cases.
In a small bowl mix the milk with the orange juice and set aside.
Using a stand mixer or beaters, cream the butter, sugar, orange zest and vanilla together until light and fluffy.
Mix the flour, baking powder, cocoa powder and pinch of salt together.
Add ½ beaten egg to the butter-sugar mixture and mix. Sift in ½ dry ingredients and gently mix with a spoon until just combined.
Repeat with the other ½ egg and flour mixture.
Add the milk-orange juice mixture and mix with a spoon until just combined. The batter should drop off the spoon, if not, add ½-1 tablespoon of orange juice or milk.
Divide the mixture between the cupcake cases, then bake for 15-19 minutes, until a skewer inserted into the middle of the cupcake comes out clean.
Leave to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
To make the buttercream/ frosting, beat the softened butter in a bowl until very soft, gradually beat in the cocoa powder and icing sugar, then gradually add the orange juice. If the mixture is too thick to pipe, add a drop more orange juice.
Spread or pipe the buttercream over the cupcakes and leave as they are, or decorate with candied orange peel, fresh orange zest, or chocolate sprinkles.
These cupcakes are best enjoyed within two days. Store at room temperature.