I use white chocolate ganache mixed with butter cream all the time. My butter cream frosting is 1/2 hi-ratio shortening to 1/2 butter. The ganache is a 3 to 1 ratio, 3 parts chocolate to 1 part heavy whipping cream. I use equal parts, 2 pounds butter cream added to 2 pounds of ganache. I warm the ganache up in the microwave for about 30 seconds and then whip it until it is very smooth. I add this to the buttercream. I apply the mixture and smooth with my flat bench scraper. I dip the blade in hot water to help get a smooth edge. I use my agbay cake leveler to level the top of the frosted cake. I wipe the blade clean with a hot clean cloth each time I pass it over the top. I get perfect clean top edges and the sides are silky smooth. This is great for a customer that does not want a fondant covered cake. When the icing has set for awhile, you can go back and smooth with a viva paper towel.