Preheat oven to 400 degrees. Mix 1/2 cup sugar and cornstarch in a 2 quart saucepan and stir in blueberries and lemon juice. Cook over medium heat, stirring frequently, until mixture boils and thickens. Boil and stir for 1 minute. Pour into ungreased 2 quart glass casserole dish and keep mixture hot in oven.
In a medium bowl, cut butter into flour, sugar, baking powder, cinnamon, and salt until mixture resembles fine crumbs. Stir in milk. Drop dough by 6 spoonfuls onto hot blueberry mixture.
Bake for 25-30 minutes until topping is golden brown. Serve warm topped with whipped cream or ice cream.