Preheat the oven to 350F and line an 8 inch square tin with parchment paper.
In a large bowl, melt together the butter and chocolate either over a saucepan of simmering water or in the microwave (stirring well every 30 seconds).
Let it cool slightly.
Add both sugars, the eggs and the vanilla to the chocolate mix and whisk with a balloon whisk until well combined.
Sift in the flour, cocoa and salt, then mix gently until just combined.
Set aside 1/4 cup of the brownie mix and pour the rest into the prepared tin.
In a medium bowl, beat together the cream cheese and sugar until smooth and creamy (about 1 minute with an electric beater).
Add the egg, sour cream and vanilla and beat until smooth.
Pour the cheesecake mixture over the brownie batter in the tin, and level out.
Use a teaspoon to dot brown batter over the top of the cheesecake mix, then run a knife through to swirl it.
Bake for 45-50 minutes, or until a toothpick comes out with just a sticky crumb or two attached.
Notes
Flour & cocoa: if using cups, spoon ingredient into cups, then level with the back of a knife so you don't add too much.