Instructions:
Mix the milk into this mixture, and then once it's mixed spread it evenly over the blueberries in the pan. Finally, mix the 1/2 cup of sugar with the cornstarch and sprinkle THAT over the mixture in the baking pan, and finally pour the boiling water over the entire thing. The original recipe suggests baking for 1 hour, but I would suggest checking it at 40 minutes. Better to have a slightly over-soft cake in this case than a burned one; this cake is pudding-ish and very soft even when it's done.
Remember to serve it warm, and don't forget the topping!