Mixed grain croissants aren't new to me. In fact I've previously tried grano arso, as well as durum. I even tried it with goat butter, which doesn't have distinct  goat flavor but does have a strange margarine-like texture.  I made a batch recent with a half-and-half wheat-and-kamut mix. A sweet, well-fed levain did the trick. This dough emerged a rocket ship following a bulk ferment and an overnight retard before introducing the butter block. Here's the final product.