Grape Tart

Make pastry cream:

Whisk together yolks and 1/2 of sugar, then whisk in flour and a pinch salt. Stir together milk and remaining sugar in a heavy saucepan and bring just to a boil over moderate heat. Whisk 1/2 of hot milk into yolks to temper and whisk yolks into milk remaining in pan. Bring custard to a boil over moderate heat and boil, whisking constantly, until very thick. Remove from heat and stir in vanilla and butter until incorporated. Chill hot pastry cream in a bowl, its surface covered with a buttered round of wax paper, until cold, about 1 hour. Make tart:

Preheat oven to 425°F while pastry cream chills. Open puff pastry sheet on a lightly floured surface and roll out to a 14- by 12-inch rectangle. Quarter pastry to form 4 rectangles. Brush 1/3 inch of edges lightly with water and fold in borders. Brush off excess flour and press borders with back of a fork to seal. Brush borders lightly with water and sprinkle with sugar. Prick insides of shells all over with fork. Bake on a large baking sheet in middle of oven until crust is golden brown, 10 to 15 minutes, and transfer to a rack. Gently flatten puff pastry inside border to deflate, and cool completely. Halve and seed grapes if necessary. Melt jelly, stirring, and toss with grapes in a bowl. Spread pastry cream in shells with a spatula and top with grapes.