In a medium shallow container, stir together the coffee, liquor or liqueur (if using), granulated sugar and salt. Set aside to cool.
Meanwhile, in a large bowl, whisk together the mascarpone and heavy cream until smooth; do not beat. Using an electric mixer, beat the cream mixture with the superfine sugar on low speed, gradually increasing the speed, but never going above medium, until it resembles marshmallow frosting; the peaks should hold, but not be stiff, and the mixture should be spreadable.
Working with one ladyfinger at a time, dip about a third of them in the coffee mixture (work quickly and do not soak them) and place them side-by-side in an 8-by-8-inch dish until the bottom is covered. Cover with half of the cream mixture. Repeat with another layer of lady fingers and the remaining cream. End with a layer of ladyfingers.
Cover the dish and refrigerate until the tiramisu is firm, about 4 hours. When ready to serve, liberally dust the top with cocoa powder. Serve with fresh berries.
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