Chocolate-Espresso Tart - bittersweet chocolate

Method

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