200g / 14 tbsp unsalted butter (1 3/4 US sticks)
200 g / 1 1/4 cups dark chocolate chips (7 oz) (Note 1)
1 cup (175g) brown sugar, loosely packed
3 eggs, lightly beaten
1 tsp vanilla extract
1/2 cup (75g) plain flour
1/4 cup (30g) cocoa powder
Pinch of salt
180g/6oz dark chocolate block/bar (optional) , chopped into chunks rather than shards, (bittersweet or semi-sweet, cooking chocolate)