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1
In bowl of electric mixer fitted with paddle attachment, cream butter and confectioners' sugar until combined, about 1 minute.
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2
Add egg yolk and vanilla and beat until smooth.
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3
Sift in flour and cocoa powder and beat on low speed until just combined.
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4
Scrape dough onto a sheet of plastic wrap and form it into a disk.
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5
Wrap and chill until firm, about 1 hour, or up to 3 days.
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6
Preheat oven to 325 degrees F. On lightly floured surface, roll tart dough to an 18 by 12-inch rectangle, 3/16-inch thick.
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7
Using a 2 1/2-inch round cutter, cut out 24 rounds of dough and press them into mini muffin tins or 2-inch tart pans, trimming away any excess dough.
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8
Prick the dough all over with a fork.
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9
Chill tart shells for 20 minutes.
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10
Line the tart shells with foil and fill with dried beans, rice, or pie weights.
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11
Bake for 15 minutes.
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12
Remove foil and weights and bake for 5 to 10 minutes longer, or until pastry looks dry and set.
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13
Transfer to a wire rack to cool.
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14
(The tart shells can be made 8 hours ahead.)
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15
Place 1/2 cup water in a large saucepan.
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16
Add sugar and corn syrup and cook mixture over medium-high heat, swirling the pan occasionally until you have a dark amber caramel, about 10 minutes.
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17
Carefully whisk in butter, cream, and creme fraiche (mixture will hiss and bubble up), whisking until smooth.
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18
(Caramel can be made up to 5 days ahead and refrigerated, then warmed over low heat or microwaved before pouring into tart shells.)
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19
Divide caramel while still warm among cooled tart shells and let sit until caramel is set, at least 45 minutes.
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20
In saucepan, bring cream to a boil.
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21
Place chopped chocolate in a bowl.
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22
Pour hot cream over chocolate and let sit for 2 minutes, then whisk until smooth.
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23
Pour some of the still-warm glaze over each tart.
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24
Let glaze set at room temperature for at least 2 hours before serving.
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25
Sprinkle with salt, if desired, just before serving.
