Preheat the oven to 190°C/374°F and line a baking tray with parchment paper.
Sift together the flour, cornstarch, baking powder, and salt.
Separate the eggs.
Beat the egg whites until frothy. Add 45g sugar and beat until stiff peaks form.
In another bowl, beat the egg yolks with 15g sugar and vanilla until pale.
Fold one-third of the whipped egg whites into the yolk mixture.
Fold the yolk mixture back into the remaining egg whites.
Add the dry ingredients in two additions and fold gently.
Pipe the batter into short lines 3 inches/7.5 cm on the tray. (2 inches or 5cm distance between each other).
Dust powdered sugar twice over the piped batter.
Bake at 180°C/356°F for 13 to 15 minutes.
Cool completely.
Mix the coffee powder, sugar, and warm water for the syrup. Cool completely.
For the filling, heat the egg yolks, sugar, and vanilla over simmering water until smooth.
Beat until pale.
Combine with mascarpone.
Whip the cream with powdered sugar until stiff peaks form.
Fold the whipped cream into the mascarpone mixture.
Dip each ladyfinger in coffee syrup for 10 to 15 seconds.
Layer the dipped ladyfingers and filling in tubs.
Repeat with a second layer.
Refrigerate for at least 3 hours or overnight.
Dust with powdered sugar and cocoa powder before serving.