Preheat the oven to 375°F. Grease a 9×13 inch pan.
In a food processor, pulse together flour, 1 cup of sugar, baking powder, cinnamon salt and lemon zest. Add the butter, egg and vanilla extra. Pulsing until dough is crumbly.
Pat half of dough into the bottom of the prepared pan, saving the remaining dough for the topping.
In a medium bowl, stir together the remaining 1/2 cup sugar, cornstarch and lemon juice. Gently mix in the dewberries.
Sprinkle the dewberry mixture evenly over the crust. Crumble remaining dough over the berry layer.
Bake in preheated oven for about 45 minutes, or until top is slightly brown (you are only looking for the top to slightly brown, if you cook for too long – the bottom will burn).
Cool completely before cutting into squares. Store in an airtight container.
