In a large bowl, or in a food processor, beat the butter and sugar on high until fully combined.
Add the flour and mix on high speed again, until the mixture resembles breadcrumbs, At this stage, you should be able to squeeze the mixture into a dough. Chill for 30 minutes.
Preheat the oven to 170C/150C Fan/Gas 3. Line an 8x8" square tin with greaseproof paper.
Remove the dough from the fridge directly into the lined tin, add in the chopped chocolate and press to the edges, ensuring the shortbread is spread evenly. It will be crumbly.
Sprinkle over the extra granulated sugar and use a sharp knife to cut the shortbread into quarters. Bake for about 40 minutes, until the top is golden.
Let cool before removing the shortbread from the tin. You may need to run a knife over the lines again to ensure clean edges.
Sprinkle over a little extra sugar, if you want, before serving!