I had never tried to combine chocolate with any citrus, except for orange. The reason is I grew up with Orange Milano cookies from Pepperidge Farm -- I thought orange and chocolate were the only possibility.
Then I took my first bite of a truffle infused with citrus. Hmm, was it lemon, grapefruit or passion fruit? It didn't matter. The combination triggered a sensory awakening.
I wanted to take cue from those pop-in-your-mouth confections to create my own dessert -- one that mimics the richness of a truffle but packed with chocolate -- blown up from bite size to cake size.
This dessert should resolve anyone's chocolate cravings -- a heart of truffle-textured chocolate surrounded by passion fruit curd and sandwiched between two classic chocolate cake layers, with everything enveloped in a ganache frosting done two ways. (More chocolate = additional antioxidant benefits if you want to play up the health angle for this decadence.) The curd lends a refreshing offset to the overall richness.
With more than 1-1/2 pounds of chocolate in this cake, I think its safe to say that it's a chocoholic's dream. If you are a purist, you can omit the passion curd and spread some ganache frosting in between the layers; but I highly recommend that you give this citrus-chocolate a try.
Passion Fruit Truffle Cake
yield One 9-inch cake
Passion Fruit Curd
1/2 cup passion fruit purée
1/2 teaspoon powdered gelatin
3/4 cup granulated sugar (5-1/4 ounces)
1/4 teaspoon fine sea salt
2 large eggs
3 large egg yolks
4 tablespoons unsalted butter (2 ounces), cut into 1/2-inch cubes and frozen
1. Measure one tablespoon passion fruit purée into small bowl; sprinkle gelatin over top. Heat remaining passion fruit purée, sugar, and salt in medium nonreactive saucepan over medium-high heat, stirring occasionally, until sugar dissolves and mixture is hot but not boiling. Whisk eggs and yolks in large nonreactive bowl. Whisking constantly, slowly pour hot lemon-sugar mixture into eggs, then return mixture to saucepan. Cook over medium-low heat, stirring constantly with heatproof spatula, until mixture registers 170 degrees on instant-read thermometer and is thick enough to leave trail when spatula is scraped along pan bottom, four to six minutes.
2. Remove pan from heat and stir in gelatin mixture until dissolved. Stir in frozen butter until incorporated. Pour filling through fine-mesh strainer into nonreactive bowl (you should have three cups). Cover surface directly with plastic wrap; refrigerate until firm enough to spread, at least four hours.
Orange-Chocolate Cake Layer
2 ounces bittersweet chocolate, finely chopped
2 tablespoons Dutch-processed cocoa, sifted
1/2 teaspoon instant espresso powder
1/4 cup boiling water
1 cups unbleached all-purpose flour (5 ounces), plus additional for dusting cake pan
1/4 teaspoon baking soda
1/2 cup (3-1/2 ounces) granulated sugar
1/3 cup (about 2-1/4 ounces) packed light brown sugar
1 tablespoons orange zest, freshly grated
1/2 teaspoon fine sea salt
6 tablespoons (3 ounces) unsalted butter, softened
2 large eggs, room temperature
1/2 teaspoon pure vanilla extract
6 tablespoons sour cream, room temperature
1. Adjust oven rack to lower-middle position; heat oven to 350℉. Combine chocolate, cocoa and espresso in small bowl; pour boiling water over and let stand to melt chocolate, about two minutes. Whisk until smooth; set aside until cooled to room temperature.
2. Spray one nine-inch-round by two-inch-high straight-sided cake pan with nonstick cooking spray; line bottom with parchment round. Spray paper round, dust pan with flour, and knock out excess. Set aside.
3. Sift flour and baking soda into medium bowl.
4. In bowl of standing mixer, beat sugars, orange zest and salt at medium-low speed until sugar is moistened, about 30 seconds. Add butter, Increase speed to medium-high and beat until mixture is light and fluffy, about three minutes, scraping down bowl with rubber spatula if necessary. Reduce speed to medium, add eggs one at a time, beating well after each addition and scraping down bowl halfway through. Beat in vanilla; increase speed to medium-high and beat until light and fluffy, about 30 seconds.
5. Add chocolate and beat until combined, about 30 seconds, scraping down bowl. With mixer running at low speed, add dry ingredients in three additions, alternating with sour cream, ending with dry ingredients. Beat in each addition until barely combined. Finish mixing by hand with rubber spatula, scraping bottom and sides of bowl, to ensure that batter is homogenous. Pour batter into prepared cake pan; spread batter to edges of pans with rubber spatula and smooth surface.
6. Bake cakes until toothpick inserted into center of cakes comes out clean, about 25-30 minutes. Cool in pans 10 minutes, then invert cakes onto greased wire rack; peel off and discard paper rounds. Cool cakes to room temperature, about one hour. (Cooled cakes can be wrapped in plastic wrap and stored at room temperature for up to one day.)
Chocolate Truffle Cake Layer
8 ounces bittersweet chocolate, chopped
1/2 cup water
3/4 cup (5-1/4 ounces) granulated sugar
1 teaspoon instant espresso powder
1/4 teaspoons fine sea salt
1 teaspoon pure vanilla extract
16 tablespoons (8 ounces/2 sticks) unsalted butter, softened and cut into 16 pieces
4 large eggs, room temperature and lightly beaten
1. Adjust oven rack to lower-middle position; heat oven to 350℉. Spray one nine-inch-round by two-inch-high straight-sided cake pan with nonstick cooking spray; line bottom with parchment round and spray parchment. Set prepared pan in large roasting pan lined with towel. Bring kettle of ayer to a boil.
2. Place chocolate in food processor fitted with metal blade. Bring water, sugar, espresso and salt to a simmer in a small saucepan over medium heat. Stir to dissolve sugar. (Make sure syrup does not come to a boil.) With food processor running, slowly pour hot syrup through the feed tube into the chocolate. Process until mixture is smooth, about one minute. Stop machine scrape side of bowl and add vanilla extract.
3. With machine running, add butter one piece at a time, process until smooth and no butter chunks remain, about one minute, scraping down bowl as needed. Pour eggs in slow stream through the feed tube into the chocolate mixture, pulse just enough to incorporate without blending in too much air. Pour batter into the prepared pan.
4. Set roasting pan on oven rack and pour enough boiling water to come about halfway up the side of cake pan. Bake for 30-35 minutes until set. Remove cake pan from water bath and set on wire rack, cool to room temperature.
5. Cover the top of the cake with plastic wrap and top with cardboard round. Invert cake, give it a gentle downward shake if necessary. Lift cake pan and peel off the parchment. Assemble immediately or cover with plastic wrap and refrigerate until ready to use.
Salty Ganache Frosting
2 cups heavy cream
1/2 teaspoon instant espresso powder
10 ounces bittersweet chocolate, finely chopped
6 ounces milk or semisweet chocolate, finely chopped
2 tablespoons light corn syrup
2 teaspoons Malden salt
1 teaspoon vanilla
1. Bring heavy cream and espresso to a full boil in a saucepan over medium heat, turn off heat. Add chocolate, and swirl pan to completely cover with cream. Slowly whisk mixture until smooth. Add corn syrup, salt and vanilla; stir until combined. Transfer to a medium-size glass bowl and let stand at cool room temperature until tick and spreadable, about 30-40 minutes.
1. Cut orange-chocolate cake layer in half horizontally. Place one cake layer on the bottom of cake stand or serving platter. Spread one cup passion fruit curd over cake layer; top with chocolate truffle layer. Top chocolate truffle cake with another cup of passion fruit curd follow by the the remaining orange-chocolate cake layer. Refrigerate for 30 minutes to set.
2. Gently spread about half of the ganache frosting (you might use less) over top and side of cake, smooth with off-set spatula,
3. Beat the remaining frosting with a hand mixer until light and fluffy, about 2 minutes. Spread whipped frosting on the side with off-set spatula. Refrigerate cake for 30 min before serving.
1. You can use ganache frosting without Step 3 or omit Step 2 and use only whipped ganache frosting, as described in Step 3.