IN MY MIND there is no real celebration unless it is capped with the sinful combination of chocolate and whipped cream. Here we expand the flavor profile, adding to the plate mascarpone and coffee elements, similar to a tiramisu. The mascarpone cream holds itself better than a simple whipped cream and brings some structure to this luscious modern dessert.
Use a large offset spatula to spread a 1-millimeter layer of tempered chocolate across each acetate sheet. Run a -inch chocolate comb widthwise through the first sheet of acetate to make 5-inch-long × -inch-wide strips. Allow the second sheet to cool for 3 minutes so that it is set but still pliable. With a chef’s knife, score the second sheet of chocolate at ¾-inch intervals widthwise to create at least twenty-four 5 × ¾-inch rectangles. For best results, allow the chocolate to rest at room temperature for 24 hours before removing from the acetate with an offset spatula.
For the Coffee-Chocolate Krispies
Place the chocolate in a heatproof bowl and microwave on high, checking every 10 seconds, until melted. Stir in the instant coffee until well incorporated. Fold in the Rice Krispies with a rubber spatula until well coated. Pour onto a parchment paper–lined tray, arranging in clumps. Cool at room temperature until hard. Store in an airtight container.
For the Coffee Crémeux
In a heatproof bowl, whisk the egg yolks with the sugar until smooth. In a medium saucepan, combine the milk, cream, and coffee extract and bring to a simmer. While whisking, gradually pour half of the hot milk into the egg yolk mixture.
Gradually whisk the mixture back into the saucepan. Return the saucepan to the heat and cook slowly, while whisking, until it reaches 185°F.
Remove from the heat and, using a hand blender, puree the chocolate in until smoooth. Strain through a fine-meshed sieve into a heatproof container. Cover with a sheet of plastic wrap and transfer to the refrigerator. Chill overnight, then transfer to a piping bag fitted with a large tip .
For the Chocolate Pâte Sablée
In a medium bowl, whisk the powdered sugar, salt, flour, cornstarch, and cocoa powder to combine. In the bowl of an electric mixer fitted with a paddle, mix the butter on medium speed until creamy. Gradually add the dry ingredients, mixing until well combined, stopping to scrape the sides of the bowl with a rubber spatula at least once. Mix in the egg yolks one at a time until well combined. Scrape the dough from the bowl, pat it into a square, and wrap in plastic wrap. Refrigerate until firm, at least 1 hour.
Preheat the oven to 350°F. Unwrap the dough and place it on a sheet of parchment paper and top with another sheet of parchment. Roll the dough through the paper to a ¼-inch-thick sheet. Transfer the dough to a baking sheet, remove the top layer of paper, and bake for 6 minutes. Remove from the oven and, with a large knife, cut the hot dough into at least six 5 × ¾-inch rectangles. Return to the oven to bake for 5 minutes, or until the rectangles start to look dry around the edges. Remove from the oven and cool at room temperature.
For the Espresso Ganache
Line an 8½ × 4½-inch loaf pan with cooking spray and acetate. Place the chocolate in a heatproof mixing bowl. In a small saucepan, combine the milk, heavy cream, and espresso, bring to a simmer, and pour over the chocolate. Rest for 30 seconds, then whisk until it is smooth and shiny. Whisk in the butter in 3 additions, until well combined. Pour into the prepared loaf pan, wrap with plastic, and transfer to the freezer until solid, about 3 hours. Loosen the sides of the frozen ganache with an offset spatula and flip it onto a cutting board. Peel away the acetate and cut into at least eight 4 × ½-inch rectangles, transfer to a tray, wrap in plastic, and refrigerate until ready to use.
For the Whipped Mascarpone Cream
Up to 1 hour before serving, transfer the mascarpone and powdered sugar to an electric stand mixer fitted with a whisk. Whip on medium speed until combined and stream in the heavy cream. Continue whipping until medium peaks form. Transfer to a piping bag fitted with a medium star tip.
For each serving, set the chocolate pâte sablée on a chilled dessert plate and set 1 tempered chocolate rectangle on top. Stack a rectangle of espresso ganache followed by another tempered chocolate rectangle on top. Pipe the coffee cream on top in a zigzag pattern to cover the rectangle and top with another tempered chocolate rectangle. Pipe mascarpone cream on top in a swirly line. Sift instant coffee on top. Pipe 2 lines of chocolate glaze on the plate, lean a few tempered chocolate strips against the bar, and set a piece of coffee-chocolate Krispies on the side.