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.

For assembly

Divide the sponge cake into thirds (three rectangles about 4-1/2×11 or 5×10 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.