Preheat the oven to 350 F or 175 C. Toss the blueberries with cinnamon and lemon juice; place in a greased 8-inch-square baking dish.
In a bowl, combine flour, sugar and baking powder; stir in milk and butter. Spoon over berries.
Combine sugar and cornstarch; sprinkle over batter.
Slowly pour boiling water over all. Bake for 45 to 50 minutes or until cake tests done.