In a large bowl, combine the egg whites and 1/4 cup sugar. Using an electric mixer with whisk attachments, beat the mixture until it holds soft peaks, about 3 to 5 minutes.
In a separate large bowl, combine the egg yolks and remaining 1/4 cup sugar. Use the electric mixer with whisk attachments (no need to wash the attachments) to whisk until pale and doubled in volume, about 3 to 5 minutes. Add the mascarpone and coffee-flavored liqueur and whisk until smooth.
Using a silicone spatula, add one third of the whipped egg whites to the mascarpone mixture and mix to incorporate. Add the second third of the egg whites and fold until just incorporated, then fold in the remaining third egg whites.
Pour the espresso into a small, shallow dish.
To build the tiramisu: dip both sides a ladyfinger in the espresso and arrange in the bottom of a 9" x 13" glass baking dish. Repeat with half the ladyfingers, arranging them tightly together to form a single layer. Pour half the mascarpone mixture on top and spread into an even layer. Repeat with the remaining espresso and ladyfingers, followed by the remaining mascarpone mixture.