Preheat Oven Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.
Separate Eggs Place egg whites in one bowl and yolks in another.
Beat egg whites until foamy, then gradually add half the sugar until stiff peaks form.
Whisk yolks with remaining sugar until thick and pale (3-4 minutes). Add vanilla.
In a separate bowl, mix gluten-free flour, salt, and cornstarch.
Gently fold dry ingredients into the yolk mixture.
Carefully fold in beaten egg whites to maintain airiness.
Transfer batter to a piping bag and pipe 3-inch strips onto baking sheets.
Dust with powdered sugar, let sit for 5 minutes, then dust again lightly.
Bake for 12-15 minutes until golden. Cool completely before using.
Beat mascarpone, sugar, egg yolks, and vanilla until smooth.
In a separate bowl, whip heavy cream until soft peaks form.
Gently fold whipped cream into mascarpone mixture in batches.
Refrigerate filling while preparing coffee soak.
Combine coffee, rum/marsala (if using), and sugar in a shallow dish. Stir until sugar dissolves.
Use an 8×8-inch dish or 9-inch round dish.
Quickly dip ladyfingers in coffee (1 second per side).
Arrange a layer of dipped ladyfingers in the dish.
Spread half the mascarpone mixture evenly over ladyfingers.
Repeat with another layer of dipped ladyfingers.
Spread remaining mascarpone mixture evenly over the top.
Use an offset spatula to smooth the top.
Cover with plastic wrap and refrigerate at least 6 hours, preferably overnight.
Before serving, dust generously with cocoa powder.
Add dark chocolate shavings if desired. Serve chilled.