4 servings | Prep Time: 1 hour | Cooking Time: 10 minutes
For the Buttercream
- 1 cup (250 ml) whole milk
- ⅔ cup (150 g) sugar, divided
- 4 egg yolks
- 3 ½ sticks (400 g) unsalted butter, at room temperature
For the Gateau
- 1 cup (250 ml) strong coffee, lukewarm
- 1/2 cup (115 ml) dark rum
- 45 Petit Beurre cookies by LU (2 200-g packages)
- Buttercream, from above
- 7 ounces (200 g) bittersweet chocolate
Fill a large mixing bowl with ice and water. Set aside. Combine the milk with half of the sugar in a small saucepan and bring to a simmer. Place the egg yolks and the remaining half of the sugar in a medium mixing bowl and whisk until pale and creamy, about 5 minutes. While continuing to whisk, slowly pour the hot milk mixture into the yolks. Pour this combined mixture back into the saucepan and cook, while stirring with a rubber spatula, until the temperature reaches 180ºF (83°C). Transfer to a clean mixing bowl and set over the ice water bowl to cool to room temperature, about 75°F (25°C). When chilled, whisk in the butter until the mixture is light and creamy.
To Assemble the Dish
Combine the coffee with the rum in a shallow bowl. Soak 2 cookies at a time in the warm coffee mixture, just long enough for them to absorb the coffee without breaking. Hold the cookies under the surface with your fingertips. It may take a couple of tries to see how much soaking is just enough; you may break a few. Reheat the coffee mixture in the microwave periodically to keep it lukewarm. Arrange the first layer of cookies on a platter in a rectangle, 3 by 3. Using a small, offset, flexible spatula, spread a thin layer of the buttercream over the cookies. Grate the chocolate on top to cover in an even layer. Repeat 4 more times to produce 5 layers of cookie, buttercream, and grated chocolate. Cover the cake with more chocolate. Spread the remaining buttercream on the sides and cover them with more chocolate, too, using the spatula to flip the grated chocolate up from the plate to cover. Refrigerate for at least 3 hours before serving.
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