Instructions

• Preheat oven to 350°F. Line the bottom of a 9-inch springform pan with parchment paper and grease the sides. Wrap the outside of the pan with 3–4 layers of 18-inch heavy-duty aluminum foil to prevent leaking.
• Melt 1 cup unsalted butter and 1 1/2 cups semi-sweet chocolate chips together until smooth (use a double boiler or microwave in 20–30 second bursts, stirring between bursts). Let the mixture cool slightly.
• In a large bowl, whisk the melted butter and chocolate until smooth. Add 1 cup granulated sugar and 1 cup packed brown sugar and whisk to combine.
• Whisk in 1 tablespoon vanilla extract. Add 4 large room-temperature eggs one at a time, whisking after each egg until incorporated.
• Add 1/2 teaspoon baking powder and 1/2 teaspoon salt, then slowly add 1 cup all-purpose flour and mix until just combined and smooth.
• Pour the brownie batter into the prepared springform pan and spread it into an even layer. Bake at 350°F for 22–24 minutes (the brownie layer may not be fully set—this is okay because it will bake again with the cheesecake).
• Remove the pan from the oven and set it aside. Reduce oven temperature to 300°F.
• While the brownie is baking or cooling, prepare the cheesecake filling: in a stand mixer or with a hand mixer, beat 24 ounces room-temperature cream cheese with 1 cup granulated sugar on low speed until smooth, scraping down the bowl as needed.
• Melt 3/4 cup semi-sweet chocolate chunks or chips (separately from the brownie chocolate) until smooth. Add the melted chocolate to the cream cheese mixture along with 3 tablespoons unsweetened cocoa powder, 1 1/2 tablespoons flour, and 2 teaspoons vanilla extract; mix on low until smooth.
• Add 4 large room-temperature eggs to the cheesecake mixture one at a time, mixing on low and scraping down the bowl after each addition.
• Add 1 cup room-temperature sour cream and beat on low speed until just combined and smooth.
• Slowly pour the cheesecake batter over the partially baked brownie layer in the springform pan, spreading it into an even layer. The pan will be very full; if you have extra batter, pour into silicone muffin cups or ramekins and bake separately.
• Place the wrapped springform pan inside a large roasting pan. Carefully pour warm (not boiling) water into the roasting pan until it reaches about halfway up the sides of the springform pan to create a water bath.
• Bake at 300°F for 1 hour and 30 minutes. The center should still be slightly jiggly when done.
• Turn off the oven and leave the cheesecake inside with the oven door fully closed for 20 minutes. After 20 minutes, crack the oven door open slightly and leave the cheesecake in the oven an additional 30 minutes or until the cheesecake feels more set.
• Remove the springform pan from the water bath, discard the aluminum foil wrapping, and allow the cheesecake to cool to room temperature. Cover and refrigerate until fully firm, at least 5–6 hours or overnight.
• Once the cheesecake is completely chilled and firm, run a thin knife around the edge, remove the springform ring, and transfer the cheesecake to a serving plate.
• Make the ganache: place 3/4 cup semi-sweet chocolate chunks or chips in a heat-safe bowl. Heat 1/3 cup heavy whipping cream until it just begins to simmer, then pour it over the chocolate. Let sit undisturbed for 1–3 minutes, then whisk until smooth.
• Spread the ganache into an even layer over the chilled cheesecake. Garnish with shaved chocolate and whipped cream, if desired. Refrigerate briefly to set the ganache before slicing.