For the sponge cake
Heat the oven to 350°F. Line a rimmed sheet pan or jelly roll pan measuring 14x11 inches or 15x10 inches (or quite close to those measurements) with kitchen parchment. Using a stand mixer with its whisk attachment or an electric mixer, beat the egg yolks, 1/3 cup of the sugar, and the hot espresso on high until thick and voluminous, at least 6 minutes. In another bowl and with clean beaters, whisk the egg whites on medium speed. When they reach the soft-peak stage, slowly add the remaining 1/3 cup sugar with the mixer running. Increase the speed to medium high and beat until the whites are glossy and smooth with peaks that hold their shape without drooping, another 1 or 2 minutes. Fold one-third of the whites into the yolk mixture to lighten them, and then gently fold in the rest of the whites. Sift the flour and salt over the top of the mixture and fold it in gently. Spread the batter evenly on the prepared sheet pan. Bake until the center of the cake springs back when lightly pressed and the cake doesn't stick to your fingers, 12 to 14 minutes. Remove from the oven and cool for 5 minutes. Run a knife around the edge of the cake and invert it onto a cooling rack. Gently peel off the parchment and allow the cake to cool completely.
For the filling
Set a pan of water to simmer and fill a large bowl with ice. In a medium stainless-steel bowl, whisk together the egg yolks, sugar, coffee liqueur, and salt. Set the bowl over, but not touching, the simmering water and whisk until the mixture thickens, the whisk leaves ribbons trailing off of it, and you can see the bottom of the bowl when you scrape the whisk along the bottom, 6 to 7 minutes. Cool the yolk mixture by setting the bowl on the ice bath and whisking the mixture occasionally. Meanwhile, whisk together the mascarpone and cream until it holds peaks. When the yolk mixture is cool, gently fold in the cream mixture until combined. Keep the mixture cool.
Divide the sponge cake into thirds (three rectangles about 4-1/2x11 or 5x10 inches). Lay one of the cake rectangles on the bottom of a rectangular pan (I use a Pyrex baking dish). Brush its top liberally with one-third of the hot espresso. Spread just under half of the mascarpone filling over the cake. Grate about one-third of the unsweetened chocolate on top of the mascarpone. Repeat with another layer of cake, espresso, mascarpone filling, and chocolate. Top with the last layer of cake, brush on the last of the coffee, spread on a very thin layer of mascarpone, and grate the rest of the chocolate on top. Refrigerate for several hours or overnight before serving. Slice and serve.
nutrition information (per serving):
26, Fat Calories
230, Saturated Fat
8, Monounsaturated Fat
34, Polyunsaturated Fat
Photo: Scott Phillips