Mix the mascarpone in a medium bowl with 3 tablespoons of the sugar. Whip the heavy cream in a large bowl with the other 4 tablespoons of sugar until fluffy.
Gently fold the sweetened mascarpone into the whipped cream.
Mix the espresso and rum in a baking dish. Dip both sides of the ladyfingers into the mixture.
Place 2 soaked ladyfingers at the base of each glass. You might need to break them in half.
Layer with half of the cream mixture. Repeat with another layer of 2 ladyfingers and finish with a layer of cream.
Sprinkle the top with cocoa powder.
Allow to chill in the fridge for at least 1 hour or overnight.