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1
Blend together the flour, cocoa powder, and salt in a bowl.
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2
Using the metal blade in a food processor, pulse the butter and the powdered sugar together until combined.
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3
Add the flour and cocoa mixture to the food processor bowl and pulse until the mixture resembles coarse meal.
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4
Add the egg and pulse until incorporated.
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5
Dump the dough onto a piece of plastic wrap or into a plastic bag and gently knead until the dough holds together.
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6
Flatten into a 6-inch disk, cover with plastic wrap (or close plastic bag), and refrigerate for 30 minutes (until just firm enough to roll).
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7
Roll out dough into a 1/8-inch-thick circle between 2 lightly floured sheets of plastic wrap.
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8
Fit dough into ungreased tart pan.
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9
Patch any holes or rips by pressing in scraps of extra dough with your fingers.
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10
Trim excess dough so that there's 1/8 inch of dough above the top edge of the tart pan (to account for shrinkage while baking).
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11
Chill for 1 hour before baking.
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12
Put oven rack in middle position and preheat oven to 375F.
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13
Lightly prick shell all over with a fork.
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14
If using pie weights: Line shell with parchment and fill with pie weights.
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15
Bake until pastry deepens in color (but does not brown), about 12 minutes.
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16
Remove parchment and bake for another 2 to 3 minutes more.
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17
If not using pie weights: Bake until pastry deepens in color (but does not brown), about 12 minutes.
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18
Check after the first 5 minutes; if center of tart puffs up, gently push down with the back of a spoon.
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19
Cool completely in the tart pan on a rack, about 20 minutes.
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20
Bring corn syrup, 3/4 cup sugar, and 2 tablespoons water to a boil in a 1 1/2- to 2-quart heavy saucepan over moderate heat, stirring until sugar is dissolved.
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21
Boil mixture, without stirring, swirling pan occasionally, until it melts into a deep golden caramel, 7 to 9 minutes.
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22
Remove saucepan from heat and add butter, vinegar, and a pinch of salt, swirling pan until butter is melted.
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23
Add 1/3 cup cream and vanilla and simmer, stirring, 1 minute (sauce will be golden brown).
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24
Cool sauce to warm.
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25
While sauce is cooling, sprinkle gelatin over remaining 2 tablespoons water in a small saucepan and let stand 1 minute to soften, then heat over low heat until dissolved.
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26
Stir into butterscotch sauce and cool to room temperature.
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27
Beat egg whites with a pinch of salt in a large bowl with an electric mixer until they hold soft peaks, then add remaining tablespoon sugar and continue to beat until whites just hold stiff glossy peaks.
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28
Fold in butterscotch sauce gently but thoroughly.
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29
Beat remaining cream with cleaned beaters until it just holds stiff peaks, then fold into butterscotch mixture gently but thoroughly.
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30
Gently pour chiffon into cooled pastry shell, letting it mound, and chill, uncovered, until set, at least 2 hours.
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31
Cook pecans in butter with salt in a small heavy skillet over moderate heat, stirring frequently, until golden, about 5 minutes.
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32
Remove from heat and cool nuts completely.
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33
Let tart stand at room temperature, about 30 minutes.
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34
Just before serving, sprinkle cooled nuts over top of tart.
