To make the allspiced butter; mix all the ingredients (except sweet potatoes, naturally) into medium bowl. Cover and refrigerate, bring to room temperature before using.
Can be made up to 3 days ahead.

Preheat oven to 375˚F.

Wash and scrub yams well, pierce all over with a fork to let steam escape.
Set yams directly on oven rack or cookie sheet covered with foil, bake until tender when pierced with fork, about 1 hour.

Using oven mits (they're hot) cut across the top of each yam squeezing them gently from the sides to expose flesh.
Spoon 1 tablespoon of the room temperature spice butter on top of each yam.

Serve, passing remaining butter.

Note: Try to pick nicely shaped, uniformly sized yams(red-skinned sweet potatoes) at your market since you will be serving them whole.