from Bon Appetit
1 cup all-purpose flour
3/4 cup sugar, divided
1/4 teaspoon kosher salt
1/2 cup (1 stick) chilled unsalted butter, cut into 1/2-inch cubes
1 10-ounce bag fresh (or frozen, thawed) cranberries
1/4 cup fresh grapefruit juice
1 teaspoon grated grapefruit zest
Preheat oven to 350°. Line an 8x8x2-inch pan with parchment, leaving a 2-inch overhang on 2 opposite sides.
Whisk flour, 1/4 cup sugar, and salt in a medium bowl. Using your fingertips, rub butter into dry ingredients until mixture 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 cooked through and pale golden, 25-30 minutes. Transfer to a wire rack. Let cool completely in pan. DO AHEAD Shortbread can be made 3 days ahead. Cover pan tightly with plastic wrap and store at room temperature.
Bring remaining 1/2 cup sugar, 1 1/2 cups cranberries, and grapefruit juice to a simmer in a small saucepan and cook, stirring occasionally, until cranberries burst and mixture is syrupy, about 8 minutes. Stir in remaining cranberries and cook until skins begin to split, about 3 minutes. Let cool. DO AHEAD Cranberry topping can be made 3 days ahead. Cover and chill.
Spread cooled cranberry topping over shortbread. Scatter zest over. Using paper overhang, lift shortbread from pan and transfer to a cutting board. Cut shortbread.