Using a paring knife, slice between the sections and membranes of the grapefruit; remove segments whole. Transfer segments to an airtight container, and chill.
Scrape vanilla seeds into a medium saucepan, and add pod. Add milk, 1/4 cup sugar, and 2 sprigs tarragon.
Place the pan over high heat. Stir to combine, and bring to a boil. Remove pan from heat; cover. Steep 20 minutes. Strain, and discard vanilla pod and tarragon sprigs.
Prepare an ice bath. In a small bowl, whisk together cornstarch and remaining 2 tablespoons sugar. Break 2 eggs into a bowl; whisk. Add cornstarch mixture; whisk thoroughly.
Bring infused milk to a boil again, and pour over egg mixture, whisking constantly. Return milk-egg mixture to the saucepan, and set over medium heat. Bring to a boil, whisking constantly. Continue whisking until very thick, about 3 minutes.
Remove pan from heat, and transfer mixture to a medium bowl. Place bowl in ice bath, and let sit, whisking occasionally, until cool. Lay a piece of plastic wrap directly on surface to prevent a skin from forming, and transfer pastry cream to the refrigerator.
Heat the oven to 350 degrees. and line a baking sheet with parchment; set aside. In a small bowl, whisk together remaining egg and 1 tablespoon heavy cream; set egg wash aside.
On a lightly floured work surface, roll out 1 sheet puff pastry just enough to smooth the creases. Cut a 5-by-10-inch rectangle, and place the rectangle on the baking sheet; chill. Using a paring knife or a leaf-shaped cookie cutter, cut 22 small leaves from the pastry scraps.
Remove rectangle from the refrigerator. Brush the bottoms of the leaves with a dab of water; place them around the perimeter of the rectangle, arranging them in an overlapping pattern. Prick the inside of the rectangle with the tines of a fork. Brush pastry with egg wash; return to refrigerator.
Repeat Steps 8 and 9 with remaining sheet puff pastry.
Bake both pieces of pastry until puffed and golden brown, about 35 minutes. Transfer shells to a wire rack to cool completely.
When ready to serve, lightly brush the remaining 4 tarragon sprigs with remaining egg wash; sprinkle with superfine sugar. Dry on unlined baking sheet, about 30 minutes.
Whip remaining 1/2 cup heavy cream until soft peaks form. Place pastry cream in a medium bowl, and whisk to loosen; fold in whipped cream. Spoon half of the pastry cream onto each pastry shell, and smooth with an offset spatula. Top with alternating pink- and white-grapefruit segments; garnish with sugared tarragon sprigs. Slice each tart into four pieces; serve.
Have been looking for various "cream fillings" for my cream puffs, so this is one I will definately try. Do you have any suggestions?