Preheat the oven to 190ºC/375ºF/gas 5. Grease a 20cm square baking tin and line with greaseproof paper. In a bowl, mix together the butter, flour, sugar and the finely grated zest of half the orange by rubbing the mixture between your thumbs and fingertips. Squash and pat into dough – don’t knead it – then push into the lined tin in a 1cm-thick layer. Prick all over with a fork, then bake for 20 minutes, or until lightly golden. Remove, sprinkle over a little extra sugar while it’s still warm, then leave to cool.
Meanwhile, melt the chocolate in a heatproof bowl over a pan of gently simmering water, then remove. Cut the shortbread into 12 finger portions, then transfer to a wire cooling rack. Drizzle with the chocolate, then finely grate over the remaining orange zest. Cut up the orange, and serve on the side!