Begin by preparing strong hot coffee (use espresso if you can). Set aside to cool.
Combine the egg yolks and ¼ cup (50g) of granulated sugar in a large bowl. Using a hand or stand mixer on medium-high speed, whip for about 5 minutes until the mixture becomes thick and pale. Be patient—keep mixing until it forms ribbons that rest on the surface for a few seconds before blending back into the mixture.
Add the cold mascarpone and vanilla to the egg yolk mixture. Mix on medium speed for about 3 minutes, until the mixture is thick and creamy. It will still be slightly soft but should hold its shape relatively well (see video for visual).
In a clean medium-sized bowl with clean whisks (see note 3), beat the egg whites on medium speed for about 30 seconds until frothy. Gradually add ½ cup (100g) of granulated sugar and continue whipping until stiff peaks form (about 5 minutes). Take care not to over-whip the egg whites.
Add half of the egg whites to the egg yolk mixture and gently fold with a spatula until just combined. Then, add the remaining egg whites and fold until fully incorporated. Be careful not to deflate the air bubbles or overmix, as this can cause the mixture to become too runny.
Assembly: At this point your coffee should be at room temperature or slightly warm (you don't want it to be hot). Carefully and quickly dip one ladyfinger at a time in the coffee for no more than 1 second on each side and place it on the bottom of a 9x13 inch dish. Repeat this step until the bottom of the dish is completely covered. You may need to break some of the ladyfingers if there are small corners you need to fill. Be careful not to leave any gaps between the lady fingers.
Next, place half of the mascarpone cream on top and spread it out evenly, making sure to reach the edges of the dish.
Next, quickly dip the remaining ladyfingers in the coffee and arrange them over the mascarpone cream, ensuring there are no gaps.
Place the remaining mascarpone cream over the top, spreading it out as evenly as you can.
Cover tightly with plastic wrap (so that it's not touching the cream) and place into the fridge for a minimum of 8 hours, ideally overnight.
Once ready, dust with cocoa powder and serve.