I've used a 100g bar of Lindt 85% dark chocolate (chopped into chunks) for the this recipe, but this can be replaced with regular chocolate chips or a different kind of chocolate (e.g. milk chocolate). Sprinkle with a pinch of salt flakes or sea salt before cooking in order to enhance the flavour even further. If you don't have 30 minutes to wait for the dough to cool in the fridge you can always pop it in the freezer for a few minutes instead.