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Mocha Buttercream and Mocha Pastry Cream

A filling of custardy pastry cream is layered within the Mocha Bubble Cake. Ultra-smooth buttercream adds a rich finishing touch.

  • prep: 45 mins
    total time: 3 hours 5 mins
  • yield: Makes 4 cups each (enough for one 8-inch layer cake)
Photography: Sang An

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Ingredients

For the Mocha Buttercream

  • 2 tablespoons instant espresso powder
  • 2 tablespoons hot water
  • 4 large egg whites, room temperature
  • 1 1/4 cups sugar
  • 3 sticks unsalted butter, room temperature, cut into small pieces
  • 5 ounces semisweet chocolate, coarsely chopped, melted and cooled

For the Mocha Pastry Cream

  • 2 1/2 teaspoons instant espresso powder
  • 2 1/2 teaspoons hot water
  • 2 large eggs, plus 2 large egg yolks
  • 1/2 cup sugar
  • 1/2 cup whole milk
  • 1/2 vanilla bean, split and scraped, pod reserved for another use
  • 1/4 cup plus 1 1/2 teaspoons cornstarch
  • Salt
  • 1 1/2 tablespoons unsalted butter, cut into small pieces
  • 1 1/2 ounces bittersweet chocolate, coarsely chopped

Cook's Note

Buttercream can be refrigerated for up to 3 days or frozen for up to 1 month. Bring to room temperature, and beat with a mixer on low speed until smooth, about 10 minutes, before using. Pastry cream can be refrigerated for up to 2 days.

Directions

  1. Step 1

    Make the mocha buttercream: Combine espresso powder and hot water in a small bowl; stir until espresso dissolves. Heat egg whites and sugar in a heatproof bowl of a mixer set over a pan of simmering water, whisking until whites are warm to the touch and sugar dissolves, 2 to 3 minutes. (Mixture should feel smooth when rubbed between 2 fingers.) Whisk egg-white mixture with a mixer on medium-high speed until stiff, glossy peaks form and mixture cools completely, about 10 minutes.

  2. Step 2

    Reduce speed to medium-low. Add butter, a few tablespoons at a time, whisking well after each addition. Switch to the paddle attachment, and reduce speed to low. Beat in espresso mixture and melted chocolate until smooth, then beat to reduce air bubbles, 3 to 5 minutes. Cover with plastic wrap, and let stand at room temperature until ready to use.

  3. Step 3

    Make the mocha pastry cream: Prepare an ice-water bath. Combine espresso powder and hot water in a small bowl; stir until espresso dissolves. Whisk eggs, egg yolks, and 1/4 cup sugar in medium bowl.

  4. Step 4

    Bring remaining 1/4 cup sugar, the milk, vanilla seeds, cornstarch, and 1/4 teaspoon salt to a boil in a medium saucepan over medium-high heat, whisking constantly. Remove from heat, and gradually whisk one-third of hot-milk mixture into egg mixture. Whisk entire mixture back into saucepan. Raise heat to medium-high, and bring to a boil, whisking constantly. Cook, continuing to whisk, until mixture is thick enough to hold its shape when lifted with a spoon, about 3 minutes.

  5. Step 5

    Remove from heat. Whisk in butter, 1 piece at a time. Whisk in espresso mixture and chopped chocolate until smooth. Strain mixture through a fine sieve into a bowl. Cover with parchment or plastic wrap, pressing it directly on surface to prevent a skin from forming. Transfer to ice-water bath; let stand until cooled, about 30 minutes. Refrigerate until chilled and firm, at least 2 hours.

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