Preheat the oven to 350°F (177°C). Line a 9x13-inch baking pan with aluminum foil or parchment paper, making sure to leave some overhang for easy removal, and spray with nonstick cooking spray. Set aside.
In a large mixing bowl, whisk together the flour, cornstarch, sugar, and salt.
Cube the butter into smaller pieces and add to the dry ingredients. Using a pastry cutter (or your hands) cut the butter into the mixture until it's crumbly and starts to come together.
Scoop the mixture into the prepared baking pan and press it down into one even layer.
Bake at 350°F (177°C) for 20-25 minutes, or just until the top is set and edges are lightly golden brown. Remove from the oven and set aside while you prepare the filling. Keep oven temperature at 350°F (177°C).