⅔cupsugarmore or less depending on sweetness of berries
1 to 2teaspoonslemon zestoptional
1tablespoonwater or milk
Preheat oven to 350 degrees. Line a 9x13 pan with foil, overhanging on the ends so you can easily lift to remove the bars from the pan. Lightly spray the foil with cooking spray.
Add the flour, salt, shortening, and butter to a large bowl and mix with a pastry blender until it looks like cornmeal with pea-sized and smaller pieces.
Place the egg yolk in a small liquid measuring cup and lightly beat with a fork. Then add milk to the mixture until it reaches 2/3 cup. Stir the milk mixture into the flour mixture.
Divide the dough into two equal portions. Lightly cover one portion with plastic wrap or other so it doesn't dry out, then roll out the other portion until it is close to 9x13 size or a little smaller. Carefully place the rolled out dough into the prepared pan as the bottom crust, gently press it out to cover the bottom of the pan, if needed. Spread crushed cornflakes over the bottom crust.
In a medium bowl, gently stir together the blackberries, sugar, cornstarch, and lemon zest (if using). Pour the berry mixture into the pan and spread on top of the cornflakes and crust.
Roll out the other portion of dough to 9x13 size and place in the pan over the berries. Lightly beat the egg white, then brush over the top crust.
Bake at 350 for 35 to 45 minutes, until crust is golden brown.
Meanwhile, in a small bowl, whisk together the powdered sugar, water or milk, and vanilla for the glaze. Pour the glaze over the bars after the bars have cooled a bit. Serve warm, at room temperature, or refrigerate and serve cold.
Apple Version: In place of the berry filling, spread 3 apples (peeled and sliced) over the crust. Then sprinkle with 1/2 cup sugar and 1/2 teaspoon cinnamon.