Here are some useful instructions for making a "root maul":
"...a mallet with a separate head and handle makes for an endgrain striking face, but what of the one-piece maul where the striking face is side grain? One-piece mauls for driving wedges are often called root mauls, because you make the striking head from the underground part of the tree. All those roots radiating from the base of the stem act to peg the striking head together and buttress the faces with their end grain.A root maul is not something you can buy. Find a tough hardwood tree about five to seven inches thick at the ground. Dig away the dirt, chopping the roots as you go. Once itís uprooted, clean it well and chop it into shape. Shape the wood while it is green and soft, but let it dry before you put it to hard use. For a two-handed maul, the head might be about ten inches long and thehandle twice as long as that."
-- from "The Woodwright's Guide" by Roy Underhill