Line an 8x8 inch baking dish with parchment paper, leaving a 1 inch overhang on two sides.
Pulse the flour, sugar and salt in a food processor until well mixed.
Add the soft butter. Pulse until the mixture looks like coarse breadcrumbs and holds together when pinched.
Press the crumbs firmly and evenly into the lined pan.
Prick the dough all over with a fork, about 1 inch apart. Score it into 2x1 inch bars with a knife.
Cover the pan and chill for at least 2 hours.
Heat the oven to 350°F (175°C). Bake on the middle rack for 30 to 35 minutes, until set and lightly golden.
Cool the pan on a wire rack for 5 minutes. Cut along the scored lines while warm. Cool completely in the pan.