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Boston Cream Pie Cake
Baked Explorations, p. 135
For the milk sponge cake
1 3/4 cups cake flour
1 1/2 tsp baking powder
3/4 tsp salt
6 Tbsp (3/4 stick) unsalted butter, cut into small pieces
3/4 cup whole milk
4 large eggs
1 1/4 cups sugar
1 1/2 tsp pure vanilla extract
For the pastry cream filling
7 large egg yolks
3/4 cup sugar
1/4 tsp salt
1/3 cup cornstarch
3 cups whole milk
3 Tbsp unsalted butter
1 Tbsp pure vanilla extract
2 Tbsp light rum
6 ounces good-quality dark chocolate (60 to 72%), finely chopped
For the chocolate glaze
5 ounces good-quality dark chocolate (60-72%), coarsely chopped
2 Tbsp dark unsweetened cocoa powder (like Valrhona)
1/4 cup sugar
1/4 cup heavy cream
1/4 cup light corn syrup
1/2 tsp vanilla extract
1 1/2 Tbsp unsalted butter
Make the milk sponge cake
Preheat oven to 325 degrees F. Butter two 8-inch round cake pans, line the bottoms iwth parchment paper, and butter the parchment. Dust the parchment with flour and knock out the excess flour.
In a large bowl, sift the cake flour, baking powder, and salt together. Set aside.
In a small saucepan over low heat, stir together the butter and milk until butter is just melted. Do not overheat. Remove the pan from the heat and set aside.
In the bowl of a standing mixer fitted with the whisk attachment, beat the eggs, sugar, and vanilla on medium speed until the mixture is pale and has tripled in volume, about 5 minutes.
Remove the bowl from the standing mixer and sprinkle a third of the flour mixture over the egg mixture. Gently fold the two together, using a rubber spatula. Add the rest of the flour mixture and fold again.
Add the warm milk mixture (if it has cooled completely, reheat it slightly) to the batter and gently fold until just incorporated.
Divide the batter between the prepared pans and smooth the tops. Bake for 20-25 minutes, rotating the pans halfway through the baking time, until a toothpick inserted in the center comes out clean. The cake might appear wobbly if you shake the pan, but if pressed gently in the middle, it should spring back.
Transfer the cake pans to a wire rack and let cool for 20 minutes. Turn the cakes out onto the rack and let them cool completely. Remove the parchment.
Make the pastry cream filling
In a large bowl, whisk together the egg yolks, half of the sugar, the salt, and cornstarch until the mixture is pale, thick and smooth.
In a medium saucepan over medium heat, stir together the milk and the remaining half of the sugar. Continue stirring until the mixture boils. Remove the pan from the heat, and pour about a third of the liquid into the egg mixture, whisking constantly. Transfer the tempered egg mixture back to the saucepan with the remaining milk mixture and, whisking constantly, bring to a boil over medium-high heat. Boil for 2 minutes.
Remove the pan from the heat and strain thought a fine-mesh sieve into a large bowl. Stir in the butter, vanilla, and rum until combined.
Scoop a third of the pastry cream into a different bowl and set it aside. Add the chocolate to the warm pastry cream remaining in the pan and stir until melted and smooth. Scoop the chocolate cream into a bowl. Once both pastry creams have cooled for about 15 minutes, wrap both bowls in plastic wrap, pressing the plastic directly onto the top of the cream to prevent a skin from forming. Refrigerate until firm, about 4 hours of overnight.
Make the chocolate glaze
Place the chocolate and cocoa powder in a large bowl and set it over a saucepan of simmering water, stirring occasionally, until completely melted and smooth.
In another small saucepan, combine the sugar, cream, corn syrup, and salt with 1/4 cup water. Place the pan over low heat and warm, without stirring, until the sugar dissolves. Increase the heat to medium and simmer for about 4 minutes, stirring frequently. Remove the pan from the heat and whisk in the vanilla and butter. Whisk to release excess heat, then add the reserved chocolate mixture and stir until smooth. Set the sauce aside; it will thicken while you assemble the cake.
Assemble the Boston Cream Pie Cake
Using a serrated knife, cut the cake layers in half horizontally. Place the first cake layer on a platter and scoop half of the chocolate pastry cream on top. Spread the cream evenly to the cake’s edges using an offset spatula. Add a second cake layer and cover that with all the vanilla cream. Add a third cake layer and cover it with the remaining chocolate cream. Place the last layer on the top and press gently with your palms to help the cake layers adhere to the pastry cream.
Very slowly, pour the thickened chocolate glaze onto the center of the top of the cake. Use an offset spatula to spread it out to the edges so that it drips down the sides of the cake. Let the sauce thicken for about 20 minutes before serving.
The cake tastes best when eaten within 24 hours. If you have leftovers, tent the entire cake in foil and refrigerate. Serve it chilled of at room temperature.
Yield: one 8-inch, 4-layer round cake