Bring a pot of water to a boil and boil the chunks just until a little tender when piercing with a fork. (Will only take a few minutes, don't boil till mushy.)
Remove potatoes from pot, but reserve some of the liquid (approximately 1/3 cup).
In skillet, melt butter.
Add at least one cup brown sugar and stir until well mixed up. (You may need more brown sugar later if it does not thicken up to make a nice glaze. You want it to be like a nice, thick syrup consistency.) Add about half to all of the reserved 1/3 cup liquid from the boiling pot. Continue stirring until thick. Again, add more brown sugar if it doesn't thicken up nicely.
Add yams or sweet potatoes to skillet and gently toss in the glaze until it thickens up and potatoes are heated through. (Keep an eye on the glaze. Stir it occasionally around the potatoes, and keep coating potatoes. If it doesn’t begin to thicken up, you'll need to add a little more brown sugar until it thickens up nicely.)