In the bowl of a stand mixer fitted with a paddle attachment, combine the cream cheese, sour cream, brown sugar, cocoa powder, butter, vanilla and salt, and beat on medium speed until smooth and creamy, 5 minutes.
Reduce the speed to medium low, pour in the melted chocolate* and mix until incorporated. Spoon the mixture into the graham cracker crust, dust with cocoa powder and sprinkle with flaky sea salt.
Refrigerate until firm, at least 30 minutes, then serve. The cheesecake can be made and chilled up to 2 days in advance. *For alternative melting instructions: Microwave morsels in a small microwave-safe bowl for 1 minute on medium-high power, then stir until smooth. Morsels may retain some of their original shape. If necessary, microwave at additional 10- to 20-second intervals, stirring until smooth.