Preheat your oven to 350°F (175°C). In a mixing bowl, combine all-purpose flour, powdered sugar, and cubed unsalted butter. Mix until the mixture resembles coarse crumbs. Press the mixture into a 9-inch tart pan. Bake for 15-20 minutes, until lightly golden; let it cool completely.

In a medium saucepan, whisk together large eggs, fresh grapefruit juice, grapefruit zest, and granulated sugar until smooth. Cook over low heat for about 10 minutes, stirring constantly until thickened. Remove from heat, stir in cornstarch and butter until combined.

Pour the warm grapefruit curd into the cooled crust, spreading it evenly. Chill in the refrigerator for at least 2 hours to allow the curd to set.

Remove the tart from the refrigerator, garnish with fresh grapefruit slices or extra zest. Slice and serve chilled.