In a mixing bowl, combine mascarpone cheese, heavy cream, powdered sugar, and vanilla extract. Beat with an electric mixer on medium speed until soft peaks form, about 3-4 minutes.

In a separate bowl, mix the cooled espresso and coffee liqueur (if using). Dip each chocolate cookie in the espresso mixture for a few seconds, then crumble them into the mascarpone mixture. Gently fold until well combined.

Cover the mixture and refrigerate for at least 1 hour, or until firm enough to handle.

Once chilled, use a small cookie scoop or your hands to form the mixture into bite-sized balls. Place the truffles on a parchment-lined baking sheet.

Melt the chopped dark chocolate in a microwave-safe bowl in 30-second intervals, stirring in between until smooth. Dip each truffle into the melted chocolate, allowing excess to drip off, then return to the baking sheet.

Sprinkle cocoa powder over the truffles while the chocolate is still wet. Refrigerate for another 30 minutes to set the chocolate.

Serve chilled or at room temperature.