01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, allowing an overhang for easy removal later.
02 - In a large bowl, cream together softened butter and granulated sugar until light and fluffy. Blend in flour and salt, mixing just until combined.
03 - Press dough evenly into the bottom of the prepared pan. Bake for 18-20 minutes until edges are lightly golden.
04 - While crust bakes, whisk together sugar and flour in a separate bowl. Add eggs, grapefruit juice, grapefruit zest, and lemon juice. Whisk until completely smooth.
05 - Pour filling mixture evenly over the hot crust immediately after removing from oven.
06 - Return pan to oven and bake for 18-22 minutes until center is just set and does not jiggle excessively.
07 - Let bars cool completely in the pan. Lift out using parchment overhang and cut into squares.
08 - Dust with powdered sugar just before serving.