I'm in the middle of making the TKOs (mock oreos) from the new Bouchon Bakery cookbook. The cookies are already baked- I even bought the black cocoa. However, the filling is a white chocolate ganache, but it won't set. Here's the recipe: 125g 35% white chocolate, 15g butter, 125g cream. Melt butter and white chocolate together, add barely simmering cream, whisk smooth, chill 4 hrs. Is there any way to tighten this up without adding cocoa butter (which I don't have)?