Preheat your oven to 350°F (175°C). In a mixing bowl, combine the crushed chocolate cookies with melted butter until evenly moistened. Press this mixture into the bottom and up the sides of a tart pan. Bake for 10 minutes, let cool completely.

In a large mixing bowl, blend together mascarpone cheese, granulated sugar, espresso powder, and vanilla extract until smooth and thick.

Once the crust has cooled, spoon the mascarpone filling into the crust and spread it evenly.

In a saucepan over low heat, combine heavy cream with chopped dark chocolate. Stir until melted and smooth.

Pour ganache over the mascarpone filling, spreading it evenly. Let tart sit to settle, then refrigerate for at least 2 hours.

After chilling, remove from the pan, slice into wedges, and optionally garnish with chocolate shavings or fresh berries.