In short, yes, you can do that. The plant has THC almost everywhere, though don't bother looking for it in roots. It's just a question of levels.
Butter isn't the greatest way to extract THC though, so the inefficiency as an extractor and the lower THC quantities in immature leaves/stems might make it difficult to get potent butter (though I have never had trouble with it myself). You might try a butane extraction or something similar. At least then you'd know what you got out of it
once you have oil, mixing it into butter and baking is easy.
When I harvest I save all the trimmings and stems, basically everything but the roots. I take all that, add in whatever males I pulled from that crop, dry it (I do this in an oven
), pulverize it (I use a mortar and pestle, but a blender works as well for everything but thick stems), and make oil. You can smoke the oil many different ways, or you can easily turn it into hash, edibles, or whatever. It's really versatile.