85g/3oz plain flour
125g/4½oz golden caster sugar
2 tbsp cocoa powder, plus a little extra for dusting, if you like
125g/4½oz dark muscovado sugar
¼ tsp fine salt
200g/7oz dark chocolate (70% cocoa solids)
100g/3½oz milk or white chocolate, or a mix of both
150g/5½oz unsalted butter, plus extra for greasing the tin
4 free-range eggs, at room temperature