To a small saucepan, add the dark rum, brown sugar, espresso powder and salt. Stir until espresso powder has dissolved; cook until reduced by almost half, about 5 to 7 minutes. Remove the pan from the heat and set aside to cool.
Meanwhile, in a bowl of a stand-up mixer (you could also do this with a hand-mixer) with the whisk attachment, add the heavy cream. Whip until light and fluffy, until it’s reached medium peaks, about 2 minutes. Scoop into a small bowl and set aside. In the same bowl of the stand-up mixer (no need to wash it out) with the paddle attachment, add the mascarpone cheese. With the mixer on low, slowly pour in the rum syrup. (It may appear curdled but it won’t affect the flavor at all—don’t stress about it!) Sift in the cocoa powder and fold it in until it’s completely combined. Lastly, fold in the whipped cream—it should be light and fluffy.
On a cake stand or large plate, arrange 6 cookies in a circle and place 1 in the center. Add a dollop of mascarpone cheese mixture to the center and spread it out into one even layer. Repeat with remaining mascarpone cheese mixture and cookies, making about 4 more layers, ending with the cheese mixture. Cover lightly with plastic wrap and transfer to the refrigerator for at least 3 hours, ideally overnight. Just before serving, dust liberally with cocoa powder.