Grease an 8-inch square pan. Pour in blueberries. Sprinkle with lemon juice. Make a batter of butter, sugar and milk, flour, salt and baking powder. Pour over berries. To prepare topping mix sugar, cornstarch and salt. Sprinkle over batter. Pour boiling water over all. Bake in preheated, 350 degree oven for 1 hour.