Preheat the oven to 325 degrees F. Line a 9x13-inch pan with nonstick foil.
Prepare the brownie batter: In a medium sized microwave-safe bowl melt the butter, then add the sugar and stir to combine. Return the mixture to the microwave briefly, just until it's hot but not bubbling. Stir in the cocoa, salt, baking powder and vanilla. Whisk in the eggs, stirring until smooth, then add the flour and chips, and stir again until smooth.
Prepare the cheesecake batter: In a medium bowl, beat the cream cheese until smooth. Add the sugar and vanilla, blending until smooth. Mix in the sour cream and eggs.
Spread three-quarters of the brownie batter into the prepared pan. Spoon the cheesecake batter over the brownie batter, smoothing it out. Drop spoonfuls of the reserved brownie mix on top. Draw a dull knife through the top third of the two batters, gently swirling it to make a swirly design.
Bake for 40-45 minutes. The top should be set and the edges should be puffy. Remove from the oven and let them cool on a rack. Chill until you're ready to cut and serve them.
