In a heatproof bowl over simmering water, whisk egg yolks and sugar constantly until mixture reaches 160°F and sugar dissolves, about 5 minutes. Remove from heat and let cool for 10 minutes. Beat in mascarpone cheese until smooth.

In a separate bowl, beat heavy cream to stiff peaks. Gently fold whipped cream into mascarpone mixture in three additions until no white streaks remain. Be careful not to deflate the mixture.

In a shallow dish, combine cooled espresso or coffee and coffee liqueur if using. Let cool to room temperature. The mixture should be strong and flavorful as it provides the key coffee taste.

Quickly dip half of the ladyfinger cookies into coffee mixture, about 1 second per side (don't oversoak or they'll get mushy). Arrange in a single layer in bottom of a 9x13-inch dish. Spread half of mascarpone cream evenly over ladyfingers.

Repeat with remaining ladyfingers and mascarpone cream. Smooth top with an offset spatula for even coverage. Cover tightly with plastic wrap, making sure wrap touches surface to prevent drying.

Refrigerate for at least 4 hours or overnight to allow flavors to meld and texture to set. Just before serving, dust generously with cocoa powder through a fine-mesh sieve and garnish with shaved dark chocolate.