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1
Whisk together yolks and 1/2 of sugar, then whisk in flour and a pinch salt.
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2
Stir together milk and remaining sugar in a heavy saucepan and bring just to a boil over moderate heat.
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3
Whisk 1/2 of hot milk into yolks to temper and whisk yolks into milk remaining in pan.
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4
Bring custard to a boil over moderate heat and boil, whisking constantly, until very thick.
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5
Remove from heat and stir in vanilla and butter until incorporated.
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6
Chill hot pastry cream in a bowl, its surface covered with a buttered round of wax paper, until cold, about 1 hour.
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7
Preheat oven to 425F while pastry cream chills.
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8
Open puff pastry sheet on a lightly floured surface and roll out to a 14- by 12-inch rectangle.
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9
Quarter pastry to form 4 rectangles.
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10
Brush 1/3 inch of edges lightly with water and fold in borders.
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11
Brush off excess flour and press borders with back of a fork to seal.
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12
Brush borders lightly with water and sprinkle with sugar.
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13
Prick insides of shells all over with fork.
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14
Bake on a large baking sheet in middle of oven until crust is golden brown, 10 to 15 minutes, and transfer to a rack.
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15
Gently flatten puff pastry inside border to deflate, and cool completely.
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16
Halve and seed grapes if necessary.
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17
Melt jelly, stirring, and toss with grapes in a bowl.
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18
Spread pastry cream in shells with a spatula and top with grapes.
