Whisk the egg yolks in a medium bowl and gradually whisk in the sugar until pale yellow in color. Fold in the mascarpone with a spatula and set aside. In another bowl, beat the egg whites until stiff and glossy. Fold the egg whites into the mascarpone mixture. Combine the coffee with a cup of boiling water. Let brew for two minutes and pour through a coffee filter to strain out the coffee grounds. Combine the coffee and the rum or Kahlua, if using, in a shallow dish or bowl. Use a bread knife to cut each lady finger in half, to make shorter lady fingers. Dip one side of each lady finger into the liquid briefly; allow any excess liquid to drip off of the ladyfingers. You’ll want four small ramekins, mason jars or glasses to make individual tiramisu cups. Arrange a layer of lady fingers in the bottom of each jar. You may need to trim them to allow them to fit. Use 1/3 of the mascarpone mixture to spread over the lady fingers in each jar. Repeat this process so that you have 3 layers of lady fingers and 3 layers of mascarpone. Cover and chill for at least 2 hours. Dust with cocoa powder just before serving; I use a small sieve to shake the cocoa over the jars, but a flour sifter would work well too.