Preheat oven to 350º F. Line a 9-inch square pan with parchment paper or grease and flour well.
In a microwave safe bowl add butter and unsweetened chocolate. Microwave in 30 second intervals, stirring each time, until melted and smooth.
Add sugar, eggs, egg yolk and vanilla. Beat until fully incorporated and combined.
Add flour, cocoa powder, and salt. Mix until just combined.
Fold in ¾ cup of chocolate chips.
In a separate bowl, using a mixer beat cream cheese until smooth. Add in sugar, egg and vanilla and beat until smooth and creamy.
Spread ⅔ds of the brownie mixture into the base of the prepared pan. Spread into a thin layer. Pour the cheesecake mixture over the top and spread evenly. Spoon the remaining brownie mixture over the top, it will not completely over the cheesecake. Sprinkle remaining chocolate chip over the top.
Bake for 30-35 minutes until the brownies are set and the edges of the cheesecake are just barely starting to turn golden.
Remove from the oven and cool completely (you can place in the fridge to chill) before slicing to serve.
