Preheat your oven to 350°F (180°C). Prepare an 11x7-inch baking dish by lightly greasing it with butter.

In a food processor, combine the all-purpose flour, powdered sugar, salt, and butter. Pulse until the mixture resembles coarse crumbs. Press it evenly into the bottom of your prepared baking dish and bake for 18 to 20 minutes.

While the crust is baking, whisk together the granulated sugar and additional flour in a mixing bowl. Add the eggs one at a time, and mix each in before adding the next. Stir in the grapefruit juice and zest until smooth.

Pour the grapefruit filling over the hot crust and bake for another 20 minutes. The edges should be set, while the center has a slight jiggle.

Let the bars cool in the pan for about 1 hour, then transfer to the refrigerator for an additional 2 hours before slicing.

Slice the cooled grapefruit bars into squares or rectangles and serve chilled, optionally dusted with powdered sugar.