250g/9oz plain flour, sifted
2 tsp baking powder, sifted
375g/13oz caster sugar
100²µ/3½´Ç³ú cocoa powder
225g/8oz icing sugar, sifted
1 tsp vanilla bean paste
200g/7oz good-quality white chocolate, broken into pieces
300²µ/10½´Ç³ú butter, plus extra for greasing
125²µ/4½´Ç³ú butter, at room temperature
3 large free-range eggs
150ml/¼ pint soured cream
150ml/¼ pint Irish stout
75ml/5 tbsp Irish cream liqueur