Preheat oven o 350 degrees F.
Line a 9x13" baking dish with foil and spray with cooking spray.
In a large bowl, combine flour, salt, sugar, and butter, and combine with a pastry blender or two forks. You can also use your fingers to get the butter evenly combined. It's ready when you can grab a fistful and it will hold together.
Press into the bottom of the prepared baking dish.
Bake in preheated oven for 10 minutes.
Meanwhile, prepare the filling.
In another large bowl, whisk together sugar and flour to combine.
Add eggs, grapefruit juice, and grapefruit zest and whisk until well combined.
Pour over baked crust.
Return to the oven and bake another 30-35 minutes, until set.
Let cool completely.
Dust with powdered sugar.
Cut into squares and enjoy!