What type of white chocolate cover do you want? If you are asking about heavy cream, I will guess it is a ganache cover you pour over the cake.
Heavy cream is whipping cream, the more butterfat the better.
To make white chocolate ganache you need almost equal parts of chocolate and cream - say 1 cup chocolate, not quite 1 cup of heavy cream. White chocolate uses a but less cream than a dark chocolate ganache would need.
Some recipes will also add a bit of butter. heat the cream to almost boiling, and pour it over the chopped chocolate, stir gently until smooth. You want to avoid making bubbles-they will show when you pour the ganache.
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