Heat the oven to 180 C, Gas 4. Lightly butter an 18x28 cm baking tin and line the base with greaseproof paper.
Set a heatproof bowl over a pan of barely simmering water, and add the butter plus 150g of the white chocolate. Whisk well until smooth then remove from the heat and allow to cool slightly.
In a separate bowl beat together the eggs, sugar and vanilla until pale. Beat in the melted white chocolate mixture.
Gently fold in the sifted flour and salt with a large metal spoon. Then add the remaining chopped chocolate, orange zest and 75g of the cranberries. Pour into the lined baking tin and sprinkle the remaining cranberries on top.
Bake for 20 minutes until the top is firm and the inside is still a bit soft, Leave to cool in the tin then cut into 12 pieces, smaller if preferred. Dust with icing sugar and serve.