Preheat your oven to 375°F (190°C) conventional (see note 1 if using a fan-forced convection oven). Lightly grease an 8x8-inch baking tin and line it with a long sheet of parchment paper so it comes up the sides with a bit of overhang—this just makes it easier to lift the bars out later. Set aside for now.
To a food processor, add all-purpose flour, cornstarch, powdered sugar, and salt. Mix for about 5 seconds to allow all the dry ingredients to combine well. Next, add in the cold unsalted butter that has been cut into small cubes and mix until the mixture resembles a sand-like consistency (see note 2 if doing this step by hand).
Transfer two-thirds of the mixture to the lined baking pan and spread it out evenly. Using your hands (or something with a flat bottom), press the crumbs into the base of the tin in an even layer. Once done, the base should look well combined and not loose.
Bake for 15 minutes, or until the top is lightly browned.
While the crust is baking, in a large bowl add the chopped strawberries along with the cornstarch, lemon juice, and sugar. Give it a good mix until everything is nicely combined, then spread evenly over the warm crust.
Lightly mix the remaining dough with your hands while gently pressing to form crumbs (the butter will be slightly softer by now, so the dough will hold together when pressed). Sprinkle the crumbs evenly over the strawberries.
Bake for another 35 minutes, or until the top is lightly browned and the strawberries are bubbling around the edges.
Let the bars cool completely in the tin (about an hour), then place in the fridge for another hour to firm up. Remove from the fridge and run a thin knife along the edges of the pan where there is no parchment to help release the bars, then use the parchment overhang to carefully lift them out. It may bend slightly as you lift, but it shouldn’t break apart. Slice and enjoy!