Preheat the oven to 170°C and line a 12-hole muffin tin with paper cases.
Process the flour, sugar, and baking powder in a food processor, then pulse in the butter.
Whisk the melted chocolate, egg, and milk together in a separate jug.
Stir the chocolate liquid into the flour mixture until just combined.
Spoon the batter into cases and bake for 15-20 minutes until a skewer comes out clean.
Cool cakes for 10 minutes, then pour a mixture of orange juice and granulated sugar over them.
Beat the butter and icing sugar for five minutes until fluffy.
Mix in the milk, melted white chocolate, and orange zest to complete the buttercream.
Pipe or spread the buttercream onto the cupcakes.
Decorate the cupcakes with orange chocolate shavings.