Preheat oven to 375°F. Grease a 9 x 13 inch pan, then line lengthwise and across with parchment for easy removal of bars from pan.
In food processor, pulse the flour, 1 cup sugar and baking powder until blended. Mix in the sea salt and lemon zest. Add the cold butter pieces to the food processor and pulse until crumbly. Add the beaten egg until combined. Dough should be crumbly like a loosely formed pastry. Pat half of the dough into the prepared pan so that the bottom is covered.
In a mixing bowl, stir together the fresly squeezed lemon juice, the remaining 1/2 cup sugar and corn starch. Gently mix in the berries. Sprinkle the berry mixture evenly over the crust and pour any remaining juice over the berries. Crumble the remaining dough over the berry layer.
Bake in preheated oven for 45 minutes to one hour or until top is slightly brown and berries are bubbling. Cool completely and refrigerate. Use parchment hanging over the side of the pan to gently lift the bars out of the pan. Cut into squares with a good serrated knife.
