Prepare the espresso, add rum to it, and mix it (it's best to pour it into a bowl). Allow it to cool down.
Separate the egg yolks from the whites.
In one bowl, whisk the egg whites with 60g of sugar and a pinch of salt until they form stiff peaks. Place this mixture in the fridge.
In another bowl, whisk the egg yolks with 70g of sugar and vanilla until they become a light color. Then, using a spatula, gently fold in the mascarpone cheese in a clockwise motion.
Add the whipped egg whites to the mascarpone mixture and gently fold them in again in a clockwise direction.
Prepare cups in which you will assemble the tiramisu. Begin by spreading a layer of the mascarpone mixture on the bottom of the cup. Then, start layering by quickly dipping each ladyfinger biscuit into the coffee mixture and placing them evenly on the cup's bottom. Add a layer of mascarpone mixture on top, spread it evenly, and sprinkle with cocoa. Repeat this process: ladyfingers soaked in coffee, mascarpone, and cocoa. Usually, 2-3 layers are enough, depending on the cup sizes.
You can sprinkle cocoa on the top layer (or do it just before serving), cover the tray with plastic wrap, and refrigerate it for at least 4 hours, preferably overnight.