I really have no advice about the buttermilk. Since so many drinks I like are fruity and/or citrusy, the idea of milk just doesn't sound awesome. LOL But I too like coffee laced with booze so I know that's more my brain objecting than anything.

You should just try it and let us know how it comes out. Worst thing that might happen is that it's not awesome. But who knows? It might be AMAZING.