A cocoa shell forms a crisp foundation for two silky-smooth fillings: an even layer of creamy, tangy mascarpone cheese and gorgeous rosettes of espresso-flavored chocolate ganache. The flavors are strong, yet not particularly sweet; if you prefer, you can mix one to two tablespoons of sugar into the mascarpone filling. To produce perfect ganache every time, make sure the mixture is at room temperature before you start to beat it with an electric mixer. Any warmer or colder, and the mixture may seize or become grainy.
Ingredients
Makes one 14-by-4-inch tart
For the Ganache
For the Crust
For the Filling
Step 1
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.
Step 2
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.
Step 3
Preheat oven to 350°F. 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.
Step 4
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).
Step 5
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.
