Preheat your oven to 190ºC (375ºF) or gas mark 5.
Prepare a 20cm square baking dish by greasing it and lining it with parchment paper.
In a mixing bowl, combine the butter, flour, sugar, and the zest from half an orange, using your thumbs and fingertips to rub the mixture together.
Form this mixture into a dough by pressing it together—avoid kneading.
Spread the dough evenly in the lined baking dish to a thickness of about 1cm.
Use a fork to prick the surface, then bake for 20 minutes or until it turns a light golden colour.
Once baked, take it out and sprinkle a little extra sugar on top while it's still warm, then let it cool.
While the shortbread cools, melt the chocolate in a heatproof bowl set over a pan of gently simmering water, then take it off the heat.
Cut the shortbread into 12 finger-sized pieces and place them on a wire rack to cool completely.
Drizzle the melted chocolate over the top and grate the remaining orange zest on top.
Serve with a cup of tea and decorated with some fresh orange slices on the side.
