Heat the oven to 180C/160C fan/gas 4. Put the sugar, flour, cocoa, pinch of salt and baking powder in a large bowl. Whisk together the eggs, vegetable oil and milk. Gradually whisk the wet ingredients into the dry until you have a smooth mixture. Fold in the dark chocolate. Put 12 cupcake cases in a 12-hole tin. Divide the mixture between the cases, then bake for 20 mins until risen. Leave to cool.
Meanwhile, make the icing. Beat the icing sugar and butter together until pale and fluffy. Whisk in 2 tbsp hot water to loosen and fold in the orange zest. Divide the icing evenly between two bowls. Add a few drops of orange food colouring to one bowl and stir until the icing turns bright orange. In the other bowl, add the cocoa powder and 2 tbsp milk, then beat until combined.
Fill one side of a piping bag (use one with a large round nozzle) with the orange icing, then fill the other side with the chocolate buttercream. Pipe on top of each cupcake. Decorate with some extra orange zest and a grating of dark chocolate.