This article says that one may make extra food on the first day of Yom Tov to be eaten on the 2nd day, as long as he tastes from each cooked dish on the day its cooked.
After tasting, does he have to leave the food in the pot, or can he take it out and store it in a container or freeze the left-overs, or is that preparing for the next day?
Also, is he forced to eat the food the 2nd day? Is it OK to change his mind and decide to keep that food for after Yom Tov? Or would that cause a "retroactive violation" in preparing on Yom Tov for a weekday?