01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, allowing edges to overhang for easy removal later.
02 - In a large bowl, cream together softened butter and granulated sugar until light and fluffy. Mix in flour and salt until just combined.
03 - Press dough evenly into pan bottom. Bake for 18-20 minutes until edges are lightly golden.
04 - While crust bakes, whisk sugar and flour in a separate bowl. Add eggs, grapefruit juice, zest, and lemon juice, whisking until smooth.
05 - Pour filling mixture 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 no longer jiggles excessively.
07 - Let cool completely in pan. Lift out using parchment edges and cut into bars.
08 - Dust with powdered sugar just before serving.