Preheat oven to 400 degrees Fahrenheit on the bake setting put oven rack in middle.
Peel and halve pears then core ea. half w/ melon-ball cutter kept in lemon-infused water until ready to cook to keep from browning.
Cut Vanilla Bean in 1/2 length-wise. Scrape seeds into sugar and mix well so that seeds are evenly distributed. (can be done 2 wks. ahead of time).
melt butter over moderate heat in 9-inch well seasoned cast iron skillet.
While the butter is melting, place the pears on towel or paper towels, cut side down.
As soon as the butter is melted, place Vanilla Bean in an 'X' pattern in middle of skillet then sprinkle in 1/2 c. Vanilla sugar so that sugar is evenly distributed.
Arrange pears, cut side up, in skillet w/wide parts at rim of skillet and narrow parts pointed to the center of the X.
Cook without stirring or shaking until sugar turns deep golden caramel, around 10 min. Watch carefully because sugar has to caramelize but not burn.
Allow pears to cool in skillet until you can safely handle the pan. Caramelized sugar can cause a nasty burn.
Roll our puff pastry dough on a lightly floured surface w/ floured rolling pin into 12" round.
Arrange puff pastry dough over the top of skillet and the caramelized pears, and trim w/ knife to fit the edge of skillet. Once trimmed, tuck edge of puff pastry dough inside rim of skillet and around the edges of pears w/ back of knife.
Bake tart until pastry is golden brn. 30-35 min. Take out of the even allow to cool.
Take a round serving plate slightly larger than skillet and place top side down over skillet. Holding the skillet and plate tightly together, invert tart onto plate, tapping the Grease bottom of skillet liberally to make sure all the tarte leaves the skillet. You can take the Vanilla Bean halves out or leave them in- just don't serve them.
Serve tarte warm w/ ice cream or whipped cream. You can reheat if you make this ahead of time.