Preheat your oven to 350°F (175°C) and prepare an 11x7-inch baking dish.

In a food processor, combine all-purpose flour, powdered sugar, salt, and cold butter. Pulse until resembling coarse crumbs. Press into the bottom of the baking dish, then bake for 18-20 minutes until golden.

In a bowl, whisk together granulated sugar and a tablespoon of flour. Add eggs one at a time, mixing well after each. Stir in grapefruit zest and juice until combined.

Pour grapefruit filling over the hot crust. Bake for an additional 20 minutes until filling is set and lightly browned.

Cool the bars on a wire rack for about 1 hour, then chill in the refrigerator for at least 2 hours before serving.

Carefully remove the bars from the dish and cut into squares. Dust with powdered sugar before serving.