Hahahaha....my 3rd time making daigaku Imo in a week! I must admit that I'm addicted to this snack. Last time, I made the caramel too watery so the pieces of sweet potatoes got soggy.
You can see the recipe here. This time, I used white sugar (not brown sugar) and I added 1 tsp of vinegar (as Yoko, my Japanese friend suggested) when making the caramel. It prevents sugar crystallization and the pieces of sweet potatoes got sticky.
In my opinion, it's just too sweet when I used white sugar instead of brown sugar. So next time, I'll use brown sugar.