In a mixing bowl, combine all-purpose flour and powdered sugar. Cut in softened unsalted butter until the mixture resembles coarse crumbs. Firmly press into a 9-inch tart pan. Blind bake at 350°F for about 20 minutes, until lightly golden brown.

In a medium saucepan, whisk grapefruit juice, zest, eggs, and granulated sugar until combined. Cook over medium heat, stirring constantly for about 10 minutes, until thickened.

Remove from heat and gradually whisk in softened unsalted butter until smooth and glossy.

Pour the grapefruit curd into the baked crust, smoothing the top. Cool at room temperature for 30 minutes, then chill for at least 4 hours.

Garnish with fresh grapefruit slices and zest before serving.