Preheat your oven to 350°F (175°C). In a mixing bowl, combine flour and powdered sugar, then cut in cold butter until the mixture resembles coarse crumbs. Press the mixture into the bottom and sides of a 9-inch tart pan. Blind bake for 20 minutes until golden brown, then let it cool.

In a saucepan over low heat, whisk together grapefruit juice, zest, sugar, and eggs. Stir continuously for about 10 minutes until the mixture thickens to a custard-like consistency. Remove from heat and mix in the butter until smooth.

Pour the grapefruit curd into the cooled tart crust, spreading it evenly. Cover with plastic wrap and refrigerate for at least 4 hours to set.

After chilling, garnish with fresh grapefruit slices and zest. Slice into wedges and serve chilled.