White ganache from cocoa butter won't set
What you're missing is solids. 17g of cream (especially when you consider the high percentage of fat in cream in relation to solids) is not going to be enough against 60g of cocoa butter. Where are you from? In the US, milk solids (in the form of dried milk powder) are cheap compared to cocoa butter. Many high quality white chocolate brands do not contain any kind of emulsifier, but all contain a significant volume of solids. Those solids are almost always milk, with the exception of a few specialized vegan varieties.

I'm wondering in what way you are trying to emulate dark chocolate, since the difference between dark chocolate and white chocolate is cocoa solids (the dark stuff)?