Prepare the pan: Heat oven to 350°F (175°C). Grease a 9-inch springform pan and line the bottom with parchment.
Make the brownie base: Melt chocolate and butter together until smooth. Mix in sugar, eggs, vanilla, cocoa powder, and salt. Spread into the pan.
Bake the base: Bake for 12–15 minutes, then let it cool while preparing the cheesecake.
Make the filling: Melt the dark chocolate and let it cool slightly. Beat cream cheese and sugar until smooth, then add eggs one at a time. Mix in vanilla, cocoa, sour cream, salt, and melted chocolate.
Bake the cheesecake: Pour the filling over the cooled base. Reduce oven temperature to 325°F (165°C) and bake for 50–65 minutes, until the edges are set and the center gently wobbles.
Cool slowly: Turn off the oven, leave the door slightly open, and let the cheesecake cool inside for about 1 hour. Cool completely before refrigerating.
Chill: Cover and refrigerate the cheesecake for at least 6 hours, preferably overnight.
Prepare the mousse: Melt chocolate and butter. Cool slightly and whisk in egg yolks. Separately whip egg whites with cream of tartar until firm peaks form.
Finish the mousse: Whip heavy cream with sugar until thick. Gently fold the egg whites and whipped cream into the chocolate mixture, then add vanilla.
Add the mousse: Spread the mousse over the chilled cheesecake while it remains in the springform pan. Refrigerate for another 2 hours.
Serve: Remove the pan ring and decorate slices with whipped cream and chocolate shavings. Use a warm, clean knife for neat slices.