Make the coffee mixture: In a shallow bowl, whisk the cooled coffee with cocoa powder and 2 tablespoons sugar until dissolved. Stir in coffee liqueur (if using) and peppermint extract.
Taste and adjust peppermint—start small; it’s potent.
Whip the cream: In a cold bowl, whip heavy cream and powdered sugar to medium-stiff peaks. Don’t overwhip.
Prepare the mascarpone: In a separate bowl, briefly whisk mascarpone with vanilla until smooth. Gently fold the whipped cream into the mascarpone in two additions until silky and uniform.
Dip the ladyfingers: One at a time, dip each ladyfinger into the coffee mixture for about 1–2 seconds per side. Do not soak; they absorb liquid fast.
First layer: Arrange a snug layer of dipped ladyfingers in an 8x8-inch dish (or similar).
Spread half the mascarpone cream over the top. Sprinkle with half the chopped dark chocolate.
Second layer: Repeat with another layer of dipped ladyfingers, then the remaining mascarpone cream. Smooth the top.
Chill: Cover and refrigerate for at least 6 hours, ideally overnight, to set and develop flavor.
Finish and serve: Just before serving, dust generously with cocoa powder.
Top with remaining chocolate shavings and a light sprinkle of crushed peppermint candies for crunch.