For the Brownie Batter

1. Preheat oven to 325°F. Line a 9×13 pan with parchment paper, coat lightly with nonstick spray and set aside.
2. In a large saucepan over medium heat, melt the butter. Once melted, add in 2 cups granulated sugar and stir constantly until the sugar is combined and begins to dissolve and gets shiny, but not bubbly. Temperature should reach 120°F. Remove the pan from the heat.
3. In a large bowl, whisk together the eggs, cocoa powder, baking powder, salt and vanilla. Mixture will be very thick. Add in the warm butter mixture and continue whisking until mostly smooth.
4. Using rubber spatula or wooden spoon, stir in the flour until combined, about 50 strokes. Set the brownie mixture aside.

For the Cheesecake Batter

1. In the bowl of your stand mixer fitted with the paddle attachment, mix the cream cheese and sugar together on medium speed for 1 minute, until combined and smooth.
2. Add in the vanilla, sour cream and eggs, and continue mixing for another minute, until smooth, scraping the sides of the bowl as necessary.

For the Assembly

1. Spread 2/3 of the brownie batter into the prepared pan. Spoon the cheesecake mixture on top of the brownie batter, and spread evenly on top. Drop by the spoonful the remainder of the brownie batter on top of the cheesecake layer. Using a butter knife swirl the brownie batter into the cheesecake mixture.
2. Bake for 35-40 minutes, until the top is set. Allow the brownies to cool completely in the pan and then cover and chill in the refrigerator for at least 2 hours. 
3. When ready to serve, cut into bars.