To make the crust, heat oven to 400º. Line the bottom of a 9-inch springform pan with a round of parchment paper. Combine the cookie crumbs and melted butter in a medium bowl and stir with a fork until all the crumbs are moistened. Transfer the crumbs to the prepared pan. Gently press the crumbs up the sides of the pan, then use a glass or other flat-bottomed object to press the crumbs into an even later on the bottom. Bake the crust for 10 minutes, then transfer to a wire rack to cool. Decrease oven temperature to 300º.

To make the filling, combine the sour cream, vanilla, peppermint and espresso powder in a small bowl. Whisk to blend and set aside. Melt the chocolate, either over a double boiler or in the microwave, and set aside to cool slightly.

In the bowl of an electric mixer, combine the cream cheese, cocoa powder and salt. Beat on medium-high speed until fluffy, about 2 minutes. Add the sugar and continue beating 2-3 minutes more, scraping the sides of the bowl as needed. Add the cooled chocolate and beat on medium-low speed. Add the sour cream mixture and mix until evenly blended. Beat in the eggs one at a time, mixing until just combined.

Pour the filling into the cooled crust. Bake for about 50-60 minutes, or until the center jiggles just slightly. Cool on a wire rack for 1-2 hours, then cover and refrigerate at least 8 hours or overnight.

To serve the cheesecake, carefully unmold the sides of the springform. Top with whipped cream and peppermint bark, if desired. Run a long, sharp knife under hot water to warm it, then dry with a towel. Cut into thin slices (this is a rich cheesecake, after all) and clean the knife, reheating as necessary, in between each slice. Serve chilled.