Preheat the oven to 400°F. Line a 9x13-inch baking pan with parchment paper so that it hangs over the two longer sides. (Note: To get the parchment to fit securely in the pan, first spray the pan with cooking spray then place the parchment paper on top.)
In a large bowl, whisk together the flour and brown sugar. Cut the butter into pieces, and using two forks or your fingers, work the butter into the flour mixture until it's the consistency of wet sand. Press the cookie dough firmly and evenly into the prepared baking pan.
Bake the shortbread for 10 to 15 minutes until it is pale golden. Remove the shortbread from the oven, place it on a cooling rack and allow it to cool completely while still in the pan. Once it has cooled, use the parchment paper overhangs to lift it out of the pan and onto your serving platter.