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1
In a food processor, pulse the confectioners' sugar with the butter and vanilla seeds until smooth.
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2
Add the egg yolk and water and process until blended.
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3
Add the flour and salt and pulse until the pastry just comes together.
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4
Pat the pastry into a disk, wrap in plastic and refrigerate until just firm enough to roll out, about 20 minutes.
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5
Preheat the oven to 350.
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6
Roll out the dough between 2 sheets of plastic wrap to an 11-inch round.
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7
Peel off the top sheet of plastic and invert the dough over a 9-inch fluted tart pan with a removable bottom.
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8
Discard the plastic and fit the pastry into the pan.
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9
Trim the overhang to 1/2 inch, fold in to reinforce the sides, and patch any holes.
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10
Prick the bottom of the tart shell with a fork and refrigerate until chilled, about 30 minutes.
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11
Line the pastry shell with foil and fill with pie weights or dried beans.
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12
Bake for 25 minutes, or until the pastry is set and the edges are golden.
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13
Carefully remove the foil and weights.
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14
Cover the rim of the tart with foil strips and bake for about 5 minutes, or until the pastry is golden and cooked through.
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15
Transfer to a rack and let cool.
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16
In a small saucepan, warm 1/4 cup of the cream with the vanilla seeds and baking soda.
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17
In a medium saucepan, cook 1/2 cup of the sugar with the corn syrup over moderate heat until a deep amber caramel forms, about 8 minutes.
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18
Remove the pan from the heat and stir in 4 tablespoons of the butter.
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19
Slowly stir in the vanilla cream in a thin stream.
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20
Cook the caramel over moderate heat until slightly thickened, about 3 minutes.
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21
Pour the hot caramel into the pastry shell and refrigerate until set, about 30 minutes.
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22
In another medium saucepan, simmer the remaining 1/2 cup of sugar with the water over high heat, stirring just until the sugar dissolves, about 2 minutes.
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23
Add the hazelnuts and cook over moderate heat, stirring frequently, until toasted and caramelized, about 10 minutes.
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24
The sugar will get very hard and crystallized after about 5 minutes, then it will melt and caramelize and the nuts will toast.
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25
Reduce the heat if the caramel darkens before the nuts are toasted.
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26
Stir in the remaining 1 tablespoon of butter.
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27
Pour the caramelized nuts onto a sheet of parchment paper and let cool completely.
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28
Transfer the nuts to a plastic bag and crush lightly with a rolling pin.
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29
In a small saucepan, heat the remaining 3/4 cup of cream until bubbles appear around the edge.
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30
Remove from the heat, add the chocolate and let stand until melted, about 5 minutes.
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31
Whisk the chocolate until smooth, then pour it over the caramel in the tart shell.
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32
Jiggle the pan to spread the chocolate in a smooth layer.
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33
Sprinkle the crushed, caramelized hazelnuts on top and refrigerate until set, about 15 minutes.
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34
Remove the tart pan and transfer the tart to a serving plate.
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35
Cut the tart into wedges and serve with whipped cream.
