1 stick unsalted butter, room temperature, cut into pieces, plus more for pan
1 tablespoon finely grated grapefruit zest
1 teaspoon vanilla extract
1/3 cup plus 1 cup powdered sugar
1 cup all-purpose flour
1/2 teaspoon kosher salt, plus more
2 tablespoons fresh grapefruit juice
Himalayan pink salt, dried hibiscus flowers, red sprinkles or sanding sugar, dried rose petals, and/or cracked pink peppercorns (for serving; optional)
Lightly butter pan. Pulse grapefruit zest, vanilla extract, 1/2 cup butter, and 1/3 cup powdered sugar in a food processor until smooth and homogenous. Add flour and 1/2 tsp. kosher salt; pulse just to combine. Press dough evenly into prepared pan, smoothing top with a flat-bottomed glass or measuring cup. Chill until firm, about 1 hour.
Preheat oven to 350°F. Prick shortbread all over with a fork 1"–2" apart, and bake until edges are golden and center is pale but firm to the touch, 15–18 minutes.
Remove band from pan and immediately cut shortbread into 16 wedges. Let cool.
Whisk grapefruit juice, a pinch of kosher salt, and remaining 1 cup powdered sugar in a small bowl until smooth. Spread a thin layer of icing over shortbreads with a small offset spatula, leaving a thin border of crust exposed. Top as desired. Let sit 10–15 minutes to allow glaze to set. Retrace cut marks.