Preheat your oven to 375°F (190°C). Make sure your oven rack is in the center position.

Prepare the crust: In a large bowl, whisk together the flour, brown sugar, and salt.

Cut in the butter: Add the cold, cubed butter to the flour mixture. Use a pastry blender or your fingertips to cut the butter into the flour until the mixture resembles coarse crumbs. You can also use a food processor for this step; just pulse until the desired consistency is achieved. The key is to keep the butter cold!

Press the crust into the pan: Press the crumb mixture evenly into the bottom of the 9x13 inch baking pan. Use the bottom of a measuring cup to help create a smooth, even surface.

Bake the crust: Bake the crust for 12-15 minutes, or until lightly golden brown. Keep a close eye on it to prevent burning.

Prepare the blueberry filling: While the crust is baking, prepare the filling. In a large bowl, gently toss together the blueberries, granulated sugar, cornstarch, lemon juice, lemon zest (if using), and cinnamon. Make sure the blueberries are evenly coated.

Pour the filling over the crust: Once the crust is lightly golden, remove it from the oven and pour the blueberry filling evenly over the top.

Bake the bars: Return the pan to the oven and bake for 35-40 minutes, or until the filling is bubbly and the crust is golden brown. The filling should be set, not runny. If the crust starts to brown too quickly, you can loosely tent the pan with foil.

Cool completely: Let the bars cool completely in the pan before cutting into squares. This is crucial! If you try to cut them while they're still warm, they'll be too soft and will fall apart. Patience is key!

Cut and serve: Once the bars are completely cool, cut them into squares and serve. Enjoy!