Step 1: Make the Brownie Crust

1. Preheat the oven: Preheat your oven to 350°F (175°C). Grease a 9-inch springform pan and line the bottom with parchment paper for easy removal.
2. Mix the brownie batter: In a large bowl, whisk together the melted butter and sugar until smooth. Add the eggs and vanilla extract, and whisk until combined. Sift in the cocoa powder, flour, and salt, then fold the mixture until just combined—don’t overmix.
3. Bake the crust: Pour the brownie batter into the prepared springform pan, spreading it into an even layer. Bake for 15–18 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Remove from the oven and let it cool slightly while you prepare the cheesecake layer.

Step 2: Make the Chocolate Cheesecake Layer

1. Prepare the cream cheese mixture: In a large mixing bowl, beat the softened cream cheese and sugar together with a hand or stand mixer until smooth and creamy (about 2 minutes).
2. Add wet ingredients: Beat in the eggs one at a time, followed by the sour cream, heavy cream, and vanilla extract. Scrape down the sides of the bowl as needed to ensure everything is fully incorporated.
3. Add cocoa and melted chocolate: Sift in the cocoa powder and beat until combined. Slowly mix in the melted chocolate, ensuring the mixture is smooth and evenly chocolatey.
4. Pour over the crust: Pour the cheesecake batter over the cooled brownie crust, smoothing it into an even layer.

Step 3: Bake the Cheesecake

1. Set up a water bath: Wrap the bottom and sides of the springform pan tightly in aluminum foil to prevent leaks. Place the pan in a large roasting pan and pour hot water into the roasting pan until it reaches halfway up the sides of the springform pan.
2. Bake the cheesecake: Bake the cheesecake at 325°F (163°C) for 50–60 minutes, or until the edges are set and the center has a slight jiggle.
3. Cool the cheesecake: Turn off the oven and leave the cheesecake inside with the door slightly ajar for 1 hour to cool gradually. Then, remove it from the water bath, discard the foil, and let it cool completely at room temperature. Cover and refrigerate for at least 6 hours or overnight.

Step 4: Make the Ganache Topping

1. Heat the cream: In a small saucepan, heat the heavy cream over medium heat until it begins to simmer. Remove from heat and pour it over the chopped chocolate in a heatproof bowl.
2. Stir until smooth: Let the mixture sit for 2–3 minutes, then stir until the chocolate is completely melted and the ganache is smooth. Stir in the vanilla extract.
3. Spread the ganache: Pour the ganache over the chilled cheesecake, spreading it evenly with an offset spatula.

Step 5: Decorate and Serve

1. Add the sprinkles: While the ganache is still soft, sprinkle the rainbow candy sprinkles evenly over the top.
2. Chill again: Return the cheesecake to the refrigerator for 15–20 minutes to set the ganache.
3. Slice and serve: Run a knife around the edge of the springform pan to release the cheesecake, then slice and enjoy!