Preheat your oven to 350°F (180°C). Line a 9-inch springform pan with parchment paper and wrap the outside with aluminum foil to prevent water from seeping in.

In a microwave-safe bowl, combine bittersweet chocolate and unsalted butter. Microwave in 30-second intervals, stirring in between until completely smooth and melted.

In a mixing bowl, whisk together granulated sugar, large eggs, and vanilla extract until frothy and pale, about 2-3 minutes.

Gently fold in the melted chocolate mixture into the egg mixture until fully combined. Then, add all-purpose flour, cocoa powder, and salt, mixing carefully until just combined.

Pour the brownie batter into the prepared springform pan, smoothing the top with a spatula. Bake for 25-30 minutes and check with a toothpick.

Lower the oven temperature to 325°F (165°C). Prepare a water bath by placing a large roasting pan with hot water on the bottom rack of the oven.

In a separate bowl, melt the semi-sweet chocolate chips until just smooth. Allow the melted chocolate to cool slightly.

In a mixer, beat the room temperature cream cheese and granulated sugar until smooth and creamy, about 3-4 minutes. Gradually mix in cooled melted chocolate, vanilla, cocoa powder, salt, and sour cream.

Add the large eggs one at a time into the cheesecake mixture, mixing on low speed until just incorporated. Pour the filling over the cooled brownie layer.

Carefully place the springform pan into the preheated water bath and bake for 60-70 minutes until edges are set and center remains slightly jiggly.

For the ganache, whisk together remaining semi-sweet chocolate chips and hot heavy cream until smooth. Chill for about an hour until thickened.

Once fully chilled overnight, spread the ganache over the cheesecake. Optionally, sprinkle candy-coated chocolate pieces on top.