Preheat oven to 350°F.

In a bowl, cream together butter and sugar until smooth. Add flour and salt and mix until a soft dough forms.

Press evenly into a parchment-lined 9×13-inch pan. Bake for 18–20 minutes, until lightly golden.

While crust bakes, whisk together sugar and flour. Add eggs and whisk until smooth.

Stir in grapefruit juice, zest, lemon juice, and salt until fully combined.

Pour filling over the hot crust. Return to oven and bake 20–25 minutes, until the center is just set.

Let cool completely, then chill for at least 1 hour for clean slices.

Dust generously with powdered sugar before serving. Enjoy!