Filling: Lightly grease a 20cm square baking dish. Place the blueberries into the baking dish then mix with vanilla and lemon juice. Sprinkle with the sugar and 1/2 teaspoon of flour, then stir in the tablespoon of melted butter. Set aside.
Topping: In a medium bowl, stir together the flour, baking powder and sugar. Rub in the butter using your fingers, or cut in with a pastry blender, until it is in small pieces. Make a well in the centre then quickly stir in the milk. Mix just until moistened. You should have a very thick batter, or very wet dough. You may need to add a splash more milk. Cover then let batter rest for 10 minutes.
To Assemble: Preheat the oven to 190 degrees C. Spoon the batter over the blueberries, leaving only a few small holes for the berries to peek through. Mix together the cinnamon and 2 teaspoons sugar then sprinkle over the top.
Bake until the top is golden brown, for 20 to 25 minutes. A knife inserted into the topping should come out clean. Let cool until just warm before serving. This can store in the refrigerator for 2 days.
For a thicker blueberry filling, add up to 1 tablespoon of flour instead of just 1/2 teaspoon.