Lorna, I am in southern California celebrating the birth of my second grandson. What an exciting, busy and wonderful event! Things are a little more settled down here than they were a few days ago. So to your question regarding "mushroom Springerle."I do believe that what your husband refers to is what I have always called a "footed Springerle". This Springerle is raised on a "foot" or a squatty stem with a larger base. This shape can be created by extra steam on the bottom surface on the cookie. To achieve this, you should let your cookies dry for 24 hours as usual, but before baking them, brush a small amount of water on the bottom of the dried cookie. This will cause a burst of steam on the bottom of the cookie, creating a foot which, yes, will make your cookie look like a mushroom. This will work best on a average size cookie, say a one and a half to two inch cookie. If you try it on a very small cookie, it will just explode the cookie; on a very large cookie, the weight of the cookie will keep this effect from happening. You will also have to bake the cookies at about 325 degrees, even if you normally bake them at a lower temperature, to get that burst of steam necessary for the "mushroom effect." You may have to practice this a bit and make adjustments for your oven. This effect is probably more easily achieved in ovens that have higher temperatures in the lower part of the oven, or direct heat from the bottom, something more likely in older baking chambers. My family's cookies did not have this shape, but I have talked with many people over the years who wanted this effect! Again, springerle cookies are so much a family tradition and it somehow becomes important to follow in the same footsteps.
Or maybe you just want to try this for the fun and experimentation....but it is a little tricky!