Preheat the oven to 170C/325F/Gas 3. Line a 12-hole muffin tin with paper cases.
Mix the flour, sugar and baking powder together in a food processor. Add the butter and pulse until combined.
Whisk the melted chocolate, egg and milk together in a jug.
Stir the chocolate mixture into the flour mixture until just combined.
Spoon the mixture into the cases and bake for 15-20 minutes, or until risen and golden-brown and a skewer inserted into the middle comes out clean. Remove from the oven and set aside to cool for 10 minutes.
Meanwhile, mix the orange juice and granulated sugar together in a bowl. Carefully pour the orange juice mixture over the warm cakes and set aside to cool completely.
For the white chocolate and orange buttercream, beat the butter in a bowl until light and fluffy. Carefully stir in the icing sugar and continue to beat for five minutes. Beat in the milk, melted chocolate and orange zest.
Decorate the cupcakes with the buttercream
Use a sharp knife to make chocolate shavings from the orange chocolate and use them to decorate the cupcakes.