Brownie crust

1. Preheat the oven to 350°F / 180°C. Line a 9-inch springform pan with parchment paper, then wrap the outside of the pan with aluminum foil. That foil barrier helps protect the pan from leaks once the water bath goes into the oven.
2. In a heatproof bowl, combine the bittersweet chocolate and unsalted butter. Microwave in 30-second bursts, stirring well after each interval, until the mixture melts completely and turns smooth. Set it aside for a few minutes so it cools slightly.
3. In a separate large mixing bowl, beat the granulated sugar, eggs, and vanilla extract with a hand mixer on medium-high speed. Keep mixing until the mixture turns pale, airy, and slightly frothy.
4. With the mixer running, slowly stream in the melted chocolate and butter mixture. Continue beating until the mixture looks fully blended and glossy.
5. Add the flour, cocoa powder, and salt. Fold them in gently until the batter no longer shows dry streaks. Stop mixing as soon as everything comes together so the crust stays tender and chewy.
6. Spread the brownie batter evenly into the prepared pan. Smooth the surface with an offset spatula or the back of a spoon.
7. Bake for 25–30 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs. Remove the pan from the oven and let the crust cool while you prepare the cheesecake filling.

Cheesecake filling

8. Lower the oven temperature to 325°F / 165°C. Place a large oven-safe skillet or casserole dish filled with warm water on the bottom rack. This creates the water bath that helps the cheesecake bake evenly and stay creamy.
9. Put the semi-sweet chocolate chips in a large microwave-safe bowl. Heat them in short bursts, stirring every 30 seconds, until fully melted. Let the chocolate cool slightly before you add it to the batter.
10. In a large bowl, beat the softened cream cheese and granulated sugar together until completely smooth. Scrape the bowl often so no cream cheese lumps hide in the batter.
11. Add the melted chocolate, vanilla extract, cocoa powder, and salt. Mix on low speed until the batter looks smooth and evenly chocolate-colored.
12. Beat in the sour cream until it disappears into the mixture. The batter should look thick, creamy, and uniform.
13. Add the eggs one at a time, mixing gently after each addition. Scrape down the bowl after the second and third eggs so the filling blends evenly.
14. Pour the cheesecake batter over the cooled brownie crust and smooth the top. Carefully place the pan in the oven on the rack directly above the water bath.
15. Bake for 60–70 minutes, or until the edges look set and the center still jiggles slightly like Jell-O when you nudge the pan. That gentle wobble means the cheesecake has baked through without drying out.
16. Turn the oven off, crack the door open, and let the cheesecake cool inside the oven for 1 hour. Then move the cheesecake to the top of the oven and let it cool there for another hour before covering it with plastic wrap.
17. Chill the cheesecake in the refrigerator overnight so the texture firms up and the flavors settle.

Chocolate ganache

18. Place the semi-sweet chocolate chips in a large bowl. Heat the heavy cream in a separate microwave-safe container for about 45 seconds, or until hot.
19. Pour the hot cream over the chocolate chips and let the bowl sit undisturbed for 1 minute. Then whisk until the mixture becomes smooth, shiny, and fully combined.
20. Cover the surface of the ganache with plastic wrap and chill it in the refrigerator for about 1 hour, or until it thickens enough to spread.
21. Once both the cheesecake and ganache have cooled completely, release the springform pan and move the cheesecake to a serving plate.
22. Spread the ganache over the top in an even layer, then finish with the candy-coated chocolate pieces. Slice and serve.