Preheat the oven to 350°F (175°C). In a food processor, crush Oreo cookies until finely ground. Melt unsalted butter and mix with crushed cookies. Press firmly into an 8x8-inch baking pan lined with parchment paper. Refrigerate for 10 minutes.

In a mixing bowl, soften cream cheese for about 30 minutes. Beat cream cheese and granulated sugar until smooth, then add cocoa powder and melted chocolate, mixing until incorporated. Add eggs one at a time, blending well after each.

Remove the crust from the refrigerator. Pour the filling over the chilled crust and spread evenly. Bake for about 45 minutes, until the center is set but slightly jiggly.

Allow the cheesecake to cool in the pan for 30 minutes at room temperature, then chill in the refrigerator for at least 4 hours.

Before serving, whip heavy cream until soft peaks form. Spread over the chilled cheesecake bars. Top with maraschino cherries and sprinkle with chocolate shavings. Slice into squares and serve chilled.