Line a 10-by-15-inch rimmed baking sheet with parchment paper; butter the paper. Press the dough evenly into the prepared pan, about 1/3 inch thick. With a knife, score the dough into 2 1/2-by-2-inch bars. Bake the shortbread for about 50 minutes, until pale golden and set. While the shortbread is still hot, cut it into bars. Transfer the baking sheet to a rack and let cool.