Preheat the oven to 375 degrees F (190 degrees C). Grease a 9×13 inch pan.

In a medium bowl, stir together 1 cup sugar, 3 cups flour, salt, lemon zest, and baking powder. Use a food processor or pastry cutter to blend in the butter, egg and vanilla. (Or if you don’t have these appliances, you can use an old fashioned fork!) Dough will be crumbly. Pat half of dough into the prepared pan.

In another bowl, stir together the sugar and cornstarch. Gently mix in the blueberries. Sprinkle the blueberry mixture evenly over the crust. Crumble remaining dough over the berry layer.

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