Whisk together the flour, cornstarch, and salt. Set aside.
In a separate bowl, combine the brown sugar and butter with your hands. Mix until combined. Add the flour mixture, and continue mixing with your hands just until combined.
Sprinkle the water over the dough. Lightly knead the dough 3 or 4 times, just until the dough is smooth and holds together.
Wrap the dough in plastic wrap. Refrigerate for 30 minutes.
Preheat oven to 325°F (160°C). Line a baking sheet with parchment paper or a silicone liner.**
Place the dough on the prepared pan. Shape the dough into an 8"x 5" rectangle about 3/8" thick. Use a fork to pierce the surface of the dough. If desired, sprinkle the top of the dough with sanding sugar.
Bake 25 to 30 minutes, or until the shortbread is set and the edges are golden brown. Cut into pieces*** and cool completely.
Notes
*Measuring accuracy is always important in baking, but perhaps even more so with shortbread. If you don't have a scale, be sure you're lightly spooning the flour into measuring cups and leveling with a knife.
**The original recipe directs you to place the dough on a baking sheet coated with cooking spray. I prefer to line my baking sheets, but either way is fine.
***The yield will vary depending on how you cut the cookies. I cut mine into wedges and got 16 cookies. Smaller squares will yield 24 pieces.