115g/4oz plain flour
150²µ/5½´Ç³ú light brown sugar
150²µ/5½´Ç³ú caster sugar
2 tbsp cocoa powder
½ tsp fine sea salt
1 tsp vanilla bean paste
175g/6oz dark chocolate, broken into small pieces
150²µ/5½´Ç³ú shortbread biscuits, roughly chopped (optional)
175g/6oz unsalted butter, plus extra for greasing
3 large free-range eggs