Whisk together the mascarpone cheese and sugar in a large mixing bowl until smooth.
Whip the heavy cream in a separate bowl until soft peaks form. Fold the whipped cream into the mascarpone mixture gently until well combined.
Combine the cooled coffee and rum/brandy in a shallow dish.
Dip each ladyfinger biscuit into the coffee and rum mixture, ensuring that both sides are saturated. In a 9x13 inch baking dish, arrange the soaked biscuits in a single layer.
Spread half of the mascarpone mixture evenly over the soaked biscuits.
Repeat with the remaining mascarpone mixture and another layer of soaked biscuits.
Refrigerate the dish for at least 4 hours, or overnight, covered in plastic wrap.
Dust the top of the tiramisu with cocoa powder and optional chocolate shavings before serving.