I've tried this one and its great, also works well with milk or dark chocolate.

Ingredients
185 g butter
1/2 cup sugar
1 tsp vanilla
3 eggs
150 g NESTLÉ PLAISTOWE White Cooking Chocolate
1 3/4 cups self-raising flour
1/4 cup milk
200 g NESTLÉ PLAISTOWE White Cooking Chocolate
1/4 cup cream


Preparation Time: 20 minutes
Cooking Time: 50 minutes
Method
Preheat oven to 180ºC.
Beat butter and sugar until creamy, beat in vanilla and eggs one at a time. Beat in NESTLÉ Plaistowe White Cooking Chocolate.
Fold in sifted flour alternately with milk.
Grease 23 cm deep round cake pan, line base with greased paper. Spoon cake mixture evenly into pan.
Bake 45-50 minutes until cooked. If cake becomes too dark, cover loosely with aluminium foil after 30 minutes.
Combine cream and melted NESTLÉ Plaistowe White Cooking Chocolate and beat until thick but runny and pour over the top. If desired serve with fresh berries

I have discovered that just swirling in melted chocolate doesn't work - what I do sometimes is divide the mixutre in 2 and add white choc to one half (with the cream) and milk choc to the other, and pour into pan to gently swirl together.