Preheat your oven to 350°F (175°C). Melt the bittersweet chocolate chips and butter over low heat or in a double boiler until smooth. Let cool slightly.
In a mixing bowl, combine the melted chocolate mixture with granulated sugar, eggs, cocoa powder, vanilla extract, and salt. Mix until fully combined.
Pour the mixture into the prepared springform pan and bake for 10-12 minutes. Let cool completely.
In a large bowl, beat together cream cheese and sugar until smooth. Add cocoa powder, vanilla extract, and bittersweet chocolate chips. Mix well.
In a separate bowl, whisk together eggs and gradually add to the cream cheese mixture, mixing until just combined.
Pour in cooled espresso and heavy cream, mixing until smooth. Fold in dark chocolate chips gently.
Pour the cheesecake filling over the cooled crust. Bake at 325°F (160°C) for about 60-70 minutes until set but slightly jiggly.
Cool the cheesecake in the oven for an hour, then refrigerate for at least 4 hours or overnight.
To prepare the topping, melt semi-sweet chocolate chips and let cool slightly. Beat cream cheese, cold heavy cream, powdered sugar, cocoa powder, and vanilla extract until soft peaks form. Fold in melted chocolate.
Spread the topping over the chilled cheesecake. Melt the dark chocolate bar and drizzle over the cheesecake for garnish.