01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Add flour and salt, mixing until just combined.
03 - Press the dough evenly into the bottom of the prepared pan. Bake for 18–20 minutes or until lightly golden at the edges.
04 - In a separate bowl, whisk together sugar and flour. Add eggs, grapefruit juice, grapefruit zest, and lemon juice. Whisk until smooth and fully combined.
05 - Pour filling over the hot crust immediately after it comes out of the oven.
06 - Return pan to the oven and bake for 18–22 minutes, or until the center is just set (it should not jiggle excessively).
07 - Remove from the oven and let cool completely in the pan. Once cooled, lift out using the parchment overhang and cut into bars.
08 - Dust with powdered sugar just before serving.