Preheat the oven to 350°F. Line an 8x8-inch baking dish with parchment paper. Set aside.
In the bowl of a stand mixer with a paddle attachment, mix the flour, cornstarch, sugar, and salt at low speed until combined (about 5 seconds).
Cut the cold Challenge butter into ½-inch cubes. Place the butter cubes in a small bowl and toss with ¼ cup of the flour mixture.
Add the floured butter (with any extra flour) to the bowl of the stand mixer. Mix on low speed until the dough is light yellow in color and small crumbs (about 4 minutes).
Remove bowl from stand mixer and use your hands to toss the mixture and rub any remaining butter bits into the flour mixture.
Press the shortbread mixture firmly and evenly into the lined pan, smoothing it with the back of a spoon. Using a fork, pierce dough at 1-inch intervals.
Bake 35-40 minutes, or until the top and edges are lightly golden brown.
Remove the shortbread from the oven. While the crust is still hot, press it down with a spatula and set it aside to cool while you prepare the caramel filling.