Preheat oven to 325°F (165°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
In a medium bowl, combine the chocolate cookie crumbs and melted butter. Press the mixture evenly into the bottom of the prepared pan. Chill in the refrigerator while preparing the filling.
In a large mixing bowl, beat the cream cheese and sugar together until smooth and creamy.
Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract, lemon zest, and lemon juice until combined.
Divide the batter in half. Mix the melted dark chocolate into one half, stirring until smooth.
Pour the lemon cheesecake mixture over the crust, spreading it evenly. Carefully spoon the chocolate cheesecake mixture on top and gently swirl with a knife to create a marbled effect.
Bake for 50-55 minutes, or until the edges are set and the center is slightly jiggly. Turn off the oven, crack the door, and let the cheesecake cool inside for 1 hour.
Refrigerate for at least 4 hours or overnight before removing from the pan.
Garnish with whipped cream, lemon slices, or chocolate shavings if desired. Serve chilled.