Preheat the oven to 180 degrees/gas mark 4. Lightly grease a 18cm/7 inch square baking tin.

 

Chop up the white chocolate and walnuts. Place the milk chocolate and the butter in a heat proof bowl, set the bowl over a saucepan of gently simmering water and heat until melted. Stir it together and let it cool slightly.

 

Whisk the eggs and sugar together in a large bowl. Then beat in the cooled chocolate mixture till combined.

 

Fold in flour, chopped walnuts and white chocolate, spoon mixture into the tin and smooth the surface.

 

Bake in the preheated oven for about 30 minutes. Cool in the tin and cut into squares.