For Puff Pastry

Cut one sheet of frozen (and defrosted) puff pastry into roughly 3x3 inch squares and set aside.

For Roasted Pear

1. Preheat oven to 375°F.
2. Place sugar in a small bowl. Using a thin, sharp knife, split the vanilla bean lengthwise and scrape out seeds. Stir the seeds into the sugar.
3. Arrange pears in a large baking dish, cut-side up. Drizzle lemon juice evenly over pears, then sprinkle with sugar. Pour water into dish. Place a dollop of butter on each pear.
4. Roast pears for 30 minutes, brushing them occasionally with pan juices. Turn pears over and continue roasting, basting occasionally, until tender and caramelized, roughly 25 to 30 minutes.
5. Cool and thinly slice each pear lengthwise.  Set aside.

For Pastry Cream

1. In a medium saucepan, combine milk, half of the sugar, vanilla bean and seeds, and salt. Cook over medium heat until mixture comes to a simmer.
2. In a medium bowl, whisk together egg yolks, cornstarch, and remaining sugar.
3. Whisking constantly, slowly pour about 1/2 cup of the hot-milk mixture into the egg-yolk mixture, 1/2 cup at a time, until it has been incorporated. Pour mixture back into the saucepan, and cook over medium-high heat, whisking constantly, until it thickens, about 2 minutes. Remove and discard vanilla bean.
4. Transfer to bowl of a stand mixer fitted with paddle attachment. Add butter and beat on medium speed until butter melts and the mixture cools.
5. Cover with plastic wrap, pressing it directly onto surface of pastry cream to prevent a skin from forming. Refrigerate until chilled, at least 2 hours.

Assembly of Tarts:

1. Preheat oven to 350°F. Line a baking sheet with parchment.  Place squares of puff pastry on parchment lined pans.
2. Whisk egg yolk lightly with a fork and brush the whole square with eggwash.
3. Spread about 1 ounce of pastry cream on each pastry square. Leave a ½ inch rim for the borders. Arrange thinly sliced pears on top of pastry cream.
4. Place baking sheet on middle shelf of preheated oven and bake for 25 minutes.
5. Remove tray from the oven, increase heat to 400°F, lightly dust tarts with sugar and return tray to  oven for a further 8 minutes. Watch that pastry does not burn, take them out earlier if needed.
6. Place tarts on a cooling rack to cool. Serve warm or cold.