Preheat the oven to 180 degrees C. Lightly grease a 28x18cm baking dish.
Spread the blueberries out to cover the entire bottom of the baking dish. Squeeze the lemon juice over them.
Batter: In a medium bowl stir together the butter and sugar until smooth. Stir in flour and baking powder alternately with the milk until smooth. Batter will be thin. Spoon over the berries and spread evenly.
Topping: In a small bowl stir together sugar, salt and cornflour. Sprinkle over the top of the batter. Dust with a pinch of cinnamon then pour the boiling water over the entire dish.
Bake for 45 minutes in the preheated oven, or until golden brown.