Butter is mostly fat which has been distilled and processed from whole milk.

For WAM purposes, it comes in three forms: soft, hard, and whipped. Butter is soft at room temperature, hard when refrigerated. It won't go bad quickly and, especially if salted, can remain at room temperature for lengthy periods. It is of course edible and is in fact an ingredient in many WAMable products. All sorts of flavors can be added to butter: https://www.tasteofhome.com/article/easy-flavored-butter-recipes/

At room temperature, butter will retain some shape and so can be worked into all sorts of nooks and crannies. It's slippery and sensuous.

However it is butter's ability to move from hard to soft which presents its most unique WAM possibilities. Think of all those obscene molds down at the local sex shop poking and prodding and then melting as you spread greasy goodness all over your lover's body. Butter works well through clothing.

If you're making a mold, don't melt the butter, just warm it enough to soften it and then put it back into the fridge. https://blog.cheesemaking.com/molding-butter-part-3/

For butter in popular culture, see Adele's Butter Dance.