Using a citrus zester, remove zest from grapefruit to equal 2 tablespoons, avoiding white pith. Remove pith from grapefruit and cut into sections over a small bowl. Finely chop grapefruit sections to equal ¾ cup. Squeeze membrane and remaining segments to yield ¼ cup juice. In a medium bowl, combine zest, chopped sections, and reserved juice. Stir in granulated sugar, brown sugar, and salt. Cover and let stand at room temperature, stirring occasionally, for 4 hours or up to overnight.
Preheat oven to 425°.
Let piecrusts stand at room temperature until slightly softened, about 5 minutes. On a lightly floured surface, roll 1 piecrust into a 12-inch circle (about ⅛ inch thick). Press into bottom and up sides of a 9-inch pie plate.
In a medium bowl, whisk together flour and cardamom. Add eggs, vanilla, and melted butter, whisking until well combined. Add flour mixture to grapefruit mixture, stirring until well combined; pour into prepared crust.
In a small bowl, whisk together egg white and 1 tablespoon water. Brush dough edges with egg wash.
On a lightly floured surface, roll remaining piecrust into a 12-inch circle. Carefully place over filling, pressing edges of top and bottom crusts together. Trim crusts to ½-to 1-inch overhang. Fold edges under and crimp as desired. Brush dough with egg wash; dust with granulated sugar. Using a sharp knife, cut 4 slits in top crust to vent.
Bake for 25 minutes, covering edges with foil to prevent excess browning and cutting slits in top, as needed. Reduce oven temperature to 350°. Bake until crust is golden brown, 20 to 25 minutes more. Let cool completely before serving. Serve with whipped cream, if desired.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/grapefruit-shaker-pie/