Brew espresso and let it cool to room temperature. Stir in Marsala if using.
Whisk egg yolks with sugar until pale and thick (3–4 minutes). Add vanilla, then mascarpone until smooth.
Whisk egg whites with salt to soft peaks. Fold gently into the mascarpone cream.
Quickly dip each ladyfinger in espresso (1–2 seconds per side). Arrange a snug layer in a 20×30 cm dish.
Spread half the mascarpone cream. Repeat with dipped ladyfingers and remaining cream.
Cover and chill at least 4 hours (better overnight). Dust with cocoa before serving.