Preheat oven to  350˚F. Linean 8 x 8 x 2-inch pan with parchment, leaving a 2-inch overhang on opposite sides.

Whisk flour, 1/4 cup sugar and salt in a medium bowl. Using your fingertips, rub butter into dry ingredients until mizture resembles coarse meal.

Transfer to prepared pan; press evenly onto bottom of pan (using the bottom of a flat measuring cup works well). Prick dough all over with a fork.

Bake shortbread until pale golden, 25 to 30 minutes. Transfer to a wire rack and allow to cool.

Bring remaining 1/2 cup sugar, 1½ cups cranberries and grapefruit juice to a simmer n a small saucepa and cook, stirring occasionally until cranberries burst and mixture is syrupy, about 8 minutes. Let cool.

To serve spread cranberry topping over shortbread. Scatter zest over top. Using paper overhang, lift shortbread from pan and transfer to a cutting board. Cut and serve.