Whip the cream. In a large bowl, combine the cold heavy whipping cream, powdered sugar, and vanilla extract. Using a hand mixer, beat on medium-high speed until soft peaks form. The cream should hold its shape when you lift the beater, but still have a gentle curve at the tip. Do not over-whip.
Prepare the mascarpone. In a separate bowl, stir the room-temperature mascarpone until smooth and completely lump-free.
Fold together. Add the whipped cream to the mascarpone in three additions, folding gently with a rubber spatula each time. Use a slow, sweeping motion — you are folding, not stirring. Stop as soon as combined. Set the mascarpone cream aside.
Make the coffee soak. Pour the cooled espresso into a shallow bowl. Stir in the liqueur if using. The coffee must be fully cooled before dipping.
Dip the ladyfingers. Hold one ladyfinger over the coffee bowl and dip each side for exactly 1–2 seconds. You want it saturated but still slightly firm. Break ladyfingers in half if needed to fit your cups.
Layer the cups. Add a small spoonful of mascarpone cream to the bottom of each cup as an anchor. Lay a single layer of dipped ladyfingers on top. Add a generous layer of mascarpone cream. Repeat with another layer of ladyfingers, then finish with a final layer of cream on top.
Chill. Cover each cup tightly with plastic wrap. Refrigerate for a minimum of 2 hours — overnight is best for flavor and texture.
Dust and serve. Just before serving, use a fine mesh sieve to dust a generous layer of unsweetened cocoa powder over each cup. Serve cold.