Preheat oven to 350°F (175°C). Line an 8×8″ pan with parchment, leaving a 2″ overhang.
In a food processor, pulse 1¾ cups flour, ⅓ cup sugar, and a pinch of salt until combined; add ¾ cup cold butter and pulse until mixture resembles coarse crumbs.
Gently press the crust into the pan and bake 15–18 minutes until edges are lightly golden.
Meanwhile, whisk 1¼ cups sugar, ⅓ cup flour, and a pinch of salt in a bowl; add eggs one at a time until smooth, then stir in ½ cup grapefruit juice and 2 tsp zest.
Pour filling over hot crust and bake 20–25 minutes until the center barely jiggles.
Cool in the pan on a wire rack for 1 hour, then refrigerate at least 2 hours before lifting out and slicing.
Notes
Store at room temperature in an airtight container for up to 1 day.Refrigerate up to 4 days or freeze individual bars for up to 1 month; thaw overnight in the refrigerator.Always wash hands and clean surfaces after handling raw eggs.