Using a hand mixer, cream the butter until light and fluffy. Add the powdered sugar and vanilla, and continue mixing until smooth and free of lumps. Mix in the grapefruit zest and flour, combining gently—either by hand or with the mixer on low speed—until a smooth, packable dough forms.
Lightly spray a shortbread mold with vegetable oil spray and firmly press the dough into the mold. Prick the surface with a fork. Bake for 30–35 minutes, or until lightly golden brown.
While the shortbread bakes, prepare the icing by whisking together the powdered sugar and grapefruit juice. Adjust the consistency by adding more powdered sugar or grapefruit juice as needed.
Allow the shortbread to cool in the pan for 10 minutes before removing. Loosen the edges with a knife, then invert the pan onto a wooden cutting board. Cut the shortbread while it is still warm.
Once completely cooled, top the cookies with the icing. Drizzle or dip as desired, using plenty of icing for best results.