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45g unsweetened cocoa powder
3 tablespoons all-purpose flour
2 tablespoons cornstarch
5 large eggs, separated
125g sugar
4 large egg yolks
1 teaspoon salt
50g canola, vegetable or other neutral
oil
3 tablespoons buttermilk
Malted Chocolate Mousse:
170g (¾ cup) whole milk
90g sugar
¼ teaspoon salt
¼ cup Horlicks or Cocoa powder
6 large egg yolks
2 gelatin sheets, soaked in cold water
until softened and drained,
or 1¼ teaspoons powdered gelatin,
softened in 1 tablespoon cold water
150g bittersweet chocolate, finely
chopped
1 cup heavy whipping cream
Vietnamese Coffee Buttercream:
½ cup (115g) unsalted butter, at room
temperature
½ cup (55g) confectioners' sugar
¼ teaspoon salt
¼ cup plus 1 tablespoon (1½ shots)
Vietnamese drip coffee or espresso
2 tablespoons sweetened condensed milk
½ teaspoon vanilla extract
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Method:
To make the chocolate cake, spray a 11 x
17-inch rimmed baking sheet with non-stick cooking spray, line with
parchment paper and spray the paper. Set aside. Sift the
cocoa, flour and cornstarch together and set aside. Put the egg
whites in the bowl of an electric mixer fitted with the whisk
attachment and whisk at medium-high speed until frothy. With the
machine running, slowly add ⅓ cup
of the sugar and continue whisking until medium-soft peaks form.
(When you lift the whisk from the mixture, a peak will form and the
top of the peak will fall back down.) Transfer the whipped
whites to a large bowl. Put the yolks in the electric mixer bowl
and whisk on medium speed until the yolks break. With the
machine running, add the salt and the remaining ⅓
cup sugar in a slow, steady stream.
Continue whisking until the mixture is pale yellow, thick and doubled
in volume, about 12 minutes. Whisk half the whites into the yolk
mixture until fully incorporated, then gently fold in the dry
ingredients until no traces of flour or cocoa remain. Add the
remaining whites to the mixture and fold gently until only a few white
streaks remain. Whisk the oil and buttermilk together until
smooth and homogenous, then fold into the batter until completely
incorporated. Spread the batter evenly in the baking sheet.
Bake for 5 minutes, then rotate the pan and bake until a tester comes
out clean, about 5 more minutes. Cool completely in the pan on a
rack. Meanwhile, make the mousse: Put the milk, sugar and
salt in a small saucepan, stir well, and set over medium heat.
Warm just until bubbles form around the edges, then whisk in the
Horlicks until completely dissolved. Remove from the heat.
Put the yolks in a large mixing bowl and whisk until the yolks break.
Add the warmed Horlicks mixture in a steady stream while whisking
constantly. Return the mixture to the saucepan and cook over low
heat, stirring constantly, for 2 minutes. Remove from the heat
and add the softened gelatin. Stir until the gelatin melts, then
stir in the chocolate until it melts completely. Let the mixture
cool to room temperature, at least an hour. Once the chocolate
has cooled, put the cream into the bowl of an electric mixer fitted
with the whisk attachment. Whisk at medium speed until medium
peaks form. (When you lift the whisk from the cream, a peak will
form, and the very tip will fall back down). Fold the cream into
the chocolate mixture. To assemble the cake: Line a 8-inch
square cake pan with plastic wrap, leaving a 10-inch overhang on all
sides. Trim 1 inch from the long side of the cake to make a 12 x
16-inch rectangle. Invert the cake onto a large work surface and
peel off the parchment paper. Cut a 4-inch-wide strip from one
long side so that you have one 4 x 16-inch rectangle and one 8 x
16-inch rectangle. Cut both rectangles crosswise in half so that
you end up with two 8-inch squares and two 4 x 8-inch rectangles.
Put one of the 8-inch squares in the bottom of the cake pan.
Spread half of the mousse on top and arrange the 2 cake rectangles
side by side on top of the mousse. Spread the remaining mousse
on the rectangles, and top with the remaining cake square. Fold
over the overhanging plastic wrap, cover with another sheet of plastic
wrap and refrigerate until set, at least 4 hours or up to 1 day.
Once the cake is set, make the buttercream: Put the butter into
the bowl of an electric mixer fitted with the paddle attachment and
beat on medium-high speed until light and fluffy, about 2 minutes.
Mix in the confectioners' sugar and salt. Scrape down the sides
and bottom of the bowl and add the coffee, condensed milk and vanilla.
Turn the speed to medium-high and mix until smooth and creamy, about 4
minutes. Invert the cake onto a cake platter and unwrap.
Use an offset spatula or butter knife to spread the buttercream evenly
over the entire surface of the cake. Refrigerate again to set
the buttercream, at least 30 minutes. Serve chilled.
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