Peel sweet potatoes. Cut crosswise into 1/2-inch-thick slices.
Layer the sweet potatoes in a large cast iron skillet. Cut each pat of butter in half, and then spread the butter pieces over the sweet potatoes. Sprinkle with sugar and a generous pinch of salt. Pour in the water and vinegar.
Place the skillet on medium heat. Bring to a simmer, then cover with a tight-fitting lid or aluminum foil (carefully, so as not to burn yourself). Simmer 12-15 minutes.
Remove the cover. Adjust heat to low, and simmer 15-20 minutes more or until the sweet potatoes are very tender, and the sauce is thick. While sweet potatoes cook, baste them occasionally with the syrup, but be careful not to break them up. Enjoy!