Step 1: Make the maple-cinnamon butter: Place the butter in a mixing bowl and whisk until light and fluffy. Whisk in the maple syrup and cinnamon. Alternatively, blend the ingredients in a mixer or food processor. Transfer the butter to a bowl or 4 ramekins and refrigerate until serving.
Step 2: To set up a charcoal grill for direct grilling, first light the charcoal in a chimney starter.
Dump the lit coals from the chimney starter into the bottom of the grill. Using a garden hoe or other long-handled implement, rake the burning coals into an even layer. Let them burn down to glowing coals. Rest the sweet potatoes on the embers. Using tongs or a hoe, rake any loose coals around the potatoes.
Step 3: Roast the sweet potatoes until the skins are charred black and the centers are soft and steamy, 40 minutes to 1 hour. Turn the potatoes with tongs from time to time to ensure even roasting. Use a slender metal skewer to test for doneness; it should insert easily.
Step 4: Remove the potatoes from the coals with tongs and brush away the ashes with a pastry brush. Let the sweet potatoes cool for a few minutes, then cut each in half lengthwise and place on plates or a platter for serving. Serve the sweet potatoes in their skins with the maple-cinnamon butter on the side. You’ll eat only the center portion of the sweet potatoes.