Stir the espresso powder and about 1 Tbsp. of the sugar into 1 cup of boiling (or at least very hot) water. Stir in the Kahlua and pour into a shallow bowl-one you can dip the ladyfingers in.
In a glass or stainless steel bowl set over a pot of simmering water, beat the egg yolks, remaining sugar and coffee or Marsala on high speed for 5-8 minutes, until the mixture pales and triples in volume. Remove from heat and beat in the mascarpone or cream cheese. It will deflate the egg yolk mixture somewhat - don't worry - just get the lumps of cheese out.
In another bowl, beat the cream until stiff peaks form. Gently fold in the mascarpone mixture.
Get out a dish that will accommodate 12 ladyfingers in the bottom, and then dip each ladyfinger into the coffee mixture, then lay it in the bottom of the dish. Lay 12 in the bottom, then top with half the mascarpone mixture. Add another layer of dipped ladyfingers, then the rest of the mascarpone mixture, smoothing the top. Sprinkle with cocoa, cover and refrigerate for at least 6 hours, or up to a day.
Let the tiramisu stand at room temperature for about 30 minutes before serving it (the flavour is better if it's not so cold), and if you need to, dust again with cocoa.