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1
Make the ganache: Put chocolate into a medium heatproof bowl.
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2
In a small saucepan, bring cream and espresso just to a boil.
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3
Pour through a fine sieve over the chocolate; discard solids.
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4
Let stand 2 minutes, then whisk until completely smooth.
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5
Let cool to room temperature, stirring occasionally, 30 to 45 minutes.
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6
Make the crust: Sift flour, salt, and cocoa into a medium bowl.
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7
With an electric mixer on medium speed, beat butter and sugar until pale and fluffy, about 4 minutes.
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8
Add egg and vanilla, and mix until combined, scraping down sides of bowl as needed.
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9
Reduce speed to low.
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10
Gradually add flour mixture in 3 batches, alternating with the cream.
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11
Shape dough into a thick rectangle; wrap in plastic.
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12
Refrigerate until firm, about 1 hour.
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13
Preheat oven to 350F.
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14
Roll out dough between 2 pieces of lightly floured parchment to a 16-by-6-inch rectangle, about 1/4 inch thick.
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15
Press dough into a 14-by-4-inch rectangular tart pan with a removable bottom.
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16
Trim excess flush with rim.
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17
Pierce bottom of shell all over with a fork.
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18
Refrigerate or freeze until firm, about 30 minutes.
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19
Bake until dry, 18 to 20 minutes.
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20
Transfer to a wire rack to cool completely; unmold.
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21
With an electric mixer on medium-high speed, beat ganache until soft peaks form, 6 to 7 minutes.
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22
Transfer to a pastry bag fitted with a 5/8-inch star tip (such as Ateco #828 or Wilton #6B).
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23
Spread mascarpone cheese evenly over bottom of tart shell with an offset spatula.
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24
Pipe rows of ganache rosettes, one next to another, on top of mascarpone to cover.
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25
Tart can be refrigerated, covered, up to 1 day before serving.
