Chop the chocolate and melt it in a water bath. Allow it to cool slightly and mix it carefully with the cream and the beaten mascarpone.
Mix the coffee with the rum. Dip each biscotti in the coffee mixture for a bit. Make a base from them, cover with a row of cream, then biscotti and finish with cream.
Sprinkle the top lightly with mocha coffee and cocoa.
Keep the chocolate tiramisu in the refrigerator for at least 1 hour. Leaving it overnight is best.