Pour the freshly-brewed espresso into a shallow bowl or serving dish. Whisk in ¼ cup granulated sugar and 2 tablespoons of rum or Kahlúa until dissolved. If you prefer a less sweet tiramisu, you can omit the sugar. Set aside.
Add the egg whites to the bowl of a stand mixer fitted with the whisk attachment, reserving the yolks for later. Whip the whites on medium-high speed until frothy and broken up, around 1 minute, then gradually pour in ¼ cup granulated sugar until incorporated. Continue to beat the meringue on high speed until it forms stiff, glossy peaks, about 5 minutes.
Using a silicone spatula, gently scrape the meringue into a separate bowl and set aside.
Place the reserved egg yolks and ¼ cup granulated sugar to the stand mixer bowl (no need to clean it), then beat on medium-high speed until the mixture becomes thick, creamy, and pale yellow, around 3 minutes.
Beat in the salt, vanilla bean paste, and the remaining 2 tablespoons of rum (if using) until combined. Scrape down the sides of the bowl, then whisk in the mascarpone cheese until creamy and evenly incorporated.
Using a silicone spatula, scrape half of the meringue into the whipped egg yolk mixture and gently fold to combine. Once incorporated, fold in the remaining half of the meringue until smooth and no white streaks remain—try not to overmix. Chill until ready to assemble.
Working one at a time, quickly dip the ladyfingers into the cooled coffee mixture (only a few seconds—too long of a soak and they will fall apart), then arrange them into a neat layer in the bottom of your serving dish.
Spread around half a layer of mascarpone cream over the ladyfingers, smoothing into an even layer.
Repeat this process once more, adding another layer of soaked biscuits and the remaining mascarpone cream to finish. Cover with plastic wrap and refrigerate for at least 4 hours or overnight.
When ready to serve, dust the tiramisu with a generous layer of cocoa powder before slicing and plating.