Warm the butter just to melt, then keep warm. Combine the remaining ingredients, then drizzle in the butter and mix well. Transfer the mixture to a container with a tight fitting lid.

The mushrooms aren't hydrated (as long as you use clarified butter or lard) so you don't have to worry about them going bad, per-se, but the flavor will be best fresh, within a couple days of making, so make it in small batches for whatever you'll need.

For the best flavor retention, refrigerate them. When the crumbs chill, they'll firm up from the butter, simply let them come to room temperature and stir, or gently warm to break them up a bit.