Preheat the oven to 375 degrees F (190 degrees C). Grease a 9x13-inch pan.

Stir flour, 1 cup sugar, and baking powder together in a medium bowl; mix in salt and cinnamon, if desired. Use a fork or pastry cutter to blend in butter and egg. Dough will be crumbly. Pat half of dough into the prepared pan.

Stir 1/2 cup sugar and cornstarch together in another bowl; gently mix in blueberries. Sprinkle blueberry mixture evenly over prepared crust. Crumble remaining dough over berry layer.

Bake in the preheated oven until top is slightly brown, about 45 minutes. Cool completely before cutting into squares.