Brownie Crust:
Preheat the oven to 350°F (180°C) and line a 9-inch springform pan with parchment paper. Wrap the bottom of the pan with aluminum foil to prevent leaks.
Melt the bittersweet chocolate and butter together in a heatproof bowl using the microwave, stirring every 30 seconds until fully melted. Allow to cool slightly.
Beat granulated sugar, eggs, and vanilla extract with a hand mixer on medium-high speed until pale and frothy.
While mixing, pour the melted chocolate and butter mixture in. Mix until fully combined.
Fold in flour, cocoa powder, and salt until no dry streaks remain. Pour the batter into the prepared pan and bake for 25-30 minutes, until a toothpick inserted into the center comes out clean. Let cool.
Cheesecake Filling:
Lower the oven temperature to 325°F (165°C). Prepare a water bath by placing a large, oven-safe dish filled with warm water on the bottom oven rack.
Melt the semi-sweet chocolate chips in a large microwave-safe bowl, stirring every 30 seconds. Set aside to cool slightly.
Beat the cream cheese and granulated sugar together until smooth and creamy. Scrape down the sides of the bowl as needed.
Add the melted chocolate, vanilla extract, cocoa powder, and salt to the cream cheese mixture and mix on low speed until smooth. Stir in the sour cream.
Add the eggs, one at a time, beating just until incorporated.
Pour the cheesecake filling over the brownie crust and smooth the top.
Bake for 60-70 minutes, until the edges are set but the center jiggles slightly. Turn off the oven, crack the door open, and let the cheesecake cool in the oven for 1 hour. Then, cool at room temperature for another hour before refrigerating overnight.
Chocolate Ganache:
Place the semi-sweet chocolate chips in a large bowl.
Heat the heavy cream in the microwave for 45 seconds, then pour it over the chocolate. Let sit for 1 minute before whisking until smooth and glossy.
Cover with plastic wrap and refrigerate for 1 hour.
Once the cheesecake is completely chilled, remove it from the springform pan, spread the ganache evenly over the top, and sprinkle with candy-coated chocolate pieces.
Slice and serve!