Preheat the oven to 325°. Line a large rimmed baking sheet with parchment paper. In a large bowl, combine the flour with the salt and the 1/4 cup of sugar. Using your fingers, rub in the butter until the mixture is the texture of sand. Gently knead the dough just until it comes together.
Transfer the dough to the baking sheet and press it into a 7-inch round, 1/3 inch thick. Prick the round all over with a fork and lightly sprinkle the top with sugar. Using the blunt side of a large knife, score the round into 8 wedges. Bake for 30 minutes, until golden brown and just cooked through. While the shortbread is still warm, cut through the score marks with a serrated knife to separate it into wedges. Serve warm or at room temperature.