I can do muffins, don't know about mushrooms.
Preference is to split muffin(s), put a thin pat of butter in the middle and put the top back on. Set on a sheet of reynold's wrap. Bake for 10 minutes at 325. If you think the sides get will get too crispy before heated, draw the sides of the tin foil up, but don't fully enclose them.
Alternately, do the butter thing, nuke for about 15 seconds. Butter should continue to melt after they come out and they should be hot. If not, zap another few seconds.