Preheat the oven to 180 degrees C. Grease and flour a 20x30cm baking dish.
In a large bowl, stir together the melted butter, milk, eggs, vanilla and 1 1/2 cups sugar substitute. Combine 3 cups of flour and baking powder; stir into the wet ingredients until just blended. Fold in the blueberries. Spread evenly in the prepared pan.
In a small bowl, stir together the brown sugar substitute, 3/4 cup of flour and cinnamon. Stir in the softened butter with a fork until the mixture is crumbly. Sprinkle over the top of the cake.
Bake for 35 to 40 minutes in the preheated oven, until a toothpick inserted into the centre of the cake comes out clean. This cake is best served warm.