225g/8oz plain flour
½ tsp baking powder
½ tsp bicarbonate of soda
30g/1oz dark muscovado sugar
150g/5oz caster sugar
50g/2oz best-quality cocoa powder, sifted
100g/4oz dark muscovado sugar
2 tsp vanilla extract
300g/10oz best-quality dark chocolate, finely chopped
125g/4½oz soft unsalted butter, plus some for greasing
175g/6oz unsalted butter, cubed
2 free-range eggs