2 tbsp plain flour
165g/5½ oz soft light brown sugar
1 tbsp cocoa powder
icing sugar, for dusting
pinch salt
1 vanilla pod, seeds only (or alternatively 2 tsp vanilla extract)
154g/5½oz pack chocolate biscuits, such as Oreos, broken into quarters
200g/7oz dark chocolate, grated or finely chopped
165g/5½oz butter, plus extra for greasing
3 free-range eggs
2 free-range egg yolks