I add melted white chocolate to my regular bc recipe, much like I would do for chocolate bc. Start with 4oz and add until it is the taste you want. 
Sometimes you will have to add more liquid to your icing because it will be drier but the melted chocolate incorprates into the icing well and doesn't dry hard like it does alone.
I have used the Baker's Chocolate squares, chocolate chips and candy melts when I didn't have either of the first two.
Here is the recipe I use and it is awesome!
6 ounces white chocolate
1 package (8 ounces) cream cheese, at room temp
1/2 stick butter at room temp
1 teaspoon vanilla
2 1/2 cups powdered sugar
Melt chocolate and let cool. Cream the cream cheese and butter, add melted choclate and blend. Add remaining ingrediants and mix on medium until fluffy, 1 to 2 minutes.
I just made White Chocolate BC. I used my regular buttercream recipe and added melted white chocolate as TexasSugar suggested.
Here is my "recipe":
2 sticks of butter
1 cup shortening
2 lbs Conf. Sugar
1 Tbls Meringue Powder
1 tsp vanilla
6oz melted white chocolate
It tastes amazing!
Thank everyone!
Caroline
I did the same thing this weekend with melted milk chocolate candy melts. My icing was sooo good.
It tastes way better than using cocoa powder.
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