According to the Nordic Recipe Archive,
"Mämmi is made with a mixture of water, rye flour and powdered rye malt, seasoned with salt and dried, powdered Seville orange peel. The mixture goes through a slow, natural sweetening process and is finally baked in oven until set. These preparations take many hours, and after baking, mämmi may yet be stored chilled for 3 to 4 days before being ready to eat."
In your typical Finnish grocery store, you will find mämmi packaged in various sizes of containers and is always refrigerated. It seems to me the most common form is found in a cake-like cardboard box, which is exactly what the mämmi we bought looked like.
When serving, you put a scoop of the cold mämmi in a bowl, sprinkle it with sugar, and top it with creme. I was expecting it to be sweet since it was a dessert after all. However, it definitely was not. If you have ever eaten 100% rye bread, you know that it has a somewhat bitter after taste. Mämmi was no different except that maybe it was even more bitter. If I eat it again, I will definitely use more sugar!