Lightly grease a 9x13-inch baking dish. Line the 18 rolls (3 across and 6 down), and set aside to let rise according to package directions.
In a saucepan, add the coconut milk, fill the can with water and add to saucepan, add the sugar. In a small bowl, mix cornstarch with 2 tablespoons of cold water until all the clumps are gone, set aside. Bring the coconut sauce to a boil add the cornstarch mixture. Simmer for about 3 minutes (sauce will thicken a little), then remove sauce from heat.
Bake rolls. After 10 minutes take rolls out of oven and pour the coconut sauce over the rolls till 1/2 to 3/4 of each bun is covered in sauce. Place the rolls back in the oven and continue baking the rolls with the sauce until the tops of the rolls are golden brown.
Remove pan and allow a few minutes to cool. Serve warm.
Left over sauce can be used to smother rolls when served individually or just make a second pan of Pani Popo.