For the brownies: Preheat the oven to 350 degrees F. Spray a 10-inch springform pan with nonstick baking spray.
Whisk together the flour, cocoa powder and salt in a medium bowl. Melt the butter and chocolate in a medium saucepan over medium heat, stirring frequently, until completely melted, then let cool for 10 minutes. Stir in the sugar and eggs. Add the flour mixture, and stir to combine. Pour brownie batter into the prepared pan and tap the pan gently to ensure the batter is even. Bake until the top is set and the edges start to pull away from the sides of the pan, about 30 minutes. Remove from the oven and let cool completely on a baking rack.
For the cheesecake: Meanwhile, beat the cream cheese with an electric mixer until smooth. Add the sugar and beat until light and fluffy, about 2 minutes. Add the sour cream and mix until just incorporated. Stir in the eggs and vanilla extract by hand, one by one, taking care not to overmix or your cheesecake will turn into a souffle.
Wrap the bottom of the pan and up the sides with foil and place in a roasting pan. Pour the cheesecake batter on top of the brownie layer and spread it evenly on top. Fill the roasting pan with water until it reaches halfway up the side of the springform pan. Bake until the center is jiggly but the surface is set, about 1 hour. Turn off the oven and leave the cheesecake inside for 30 minutes.
Remove the cheesecake from the oven and run a sharp knife around the edges of the cake to release from the pan. Let rest on a baking rack until completely cool. Cover and refrigerate for 8 hours or up to overnight before serving.
Slice and garnish each slice with chocolate syrup and whipped cream.
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