• Preheat oven to 375°F. Grease a 9x13-inch pan.

Make the Crumb Mixture:

• In the bowl of a food processor, add flour, baking powder, salt, sugar, and lemon zest, and pulse briefly to mix.
  3 cups all purpose flour, 1 teaspoon baking powder, ¼ teaspoon salt, 1 cup sugar, zest of 1 lemon
• Add cold butter (cut into tablespoon-sized chunks) and the egg, and pulse until the mixture is crumbly yet sticks together. Alternatively, cut the butter and egg into the dry ingredients using a fork or a pastry blender.  Pat half the dough into the prepared pan for the bottom crust.
  1 cup cold butter, 1 egg

Make the Filling:

• In a medium bowl, stir together the sugar, cornstarch, and lemon juice.
  juice of 1 lemon, ½ cup sugar, 4 teaspoons cornstarch
• Gently mix in the blueberries.
  4 cups fresh blueberries

Assemble and Bake the Bars:

• Sprinkle the blueberry mixture evenly over the crust.
• Crumble the remaining dough on top of the blueberries.
• Bake for 45 minutes or until top is slightly brown. Cool completely before cutting into squares. I typically cool the bars in the pan to room temperature then place the pan in the fridge overnight and cut them the next day. I think this is a bit easier.