Preheat oven to 350 degrees F. Line an 8-inch square metal pan with foil and spray foil with cooking spray or line with parchment paper.
Beat softened cream cheese and sugar together in a medium bowl. When smooth, beat in egg and vanilla. Set aside.
Melt chocolate and butter together in a saucepan.
Whisk and sugar together in a medium bowl. When egg mixture is light and frothy, whisk in vanilla, salt and baking powder. Stir in the melted chocolate/butter mixture. Add flour and stir until smooth.
Make sure brownie batter is cool, then pour half of brownie batter into pan. Pour cream cheese batter over brownie batter. Pour remaining brownie batter over top, marbling or making a design. Or you can just leave it as a slab of chocolate.
Bake on center rack for 35 minutes (see note). Let cool to room temperature on cooling rack, then chill for at least an hour (preferably longer) before serving. A longer chill will make for a smoother filling.
Cut into 9, 12 or 16 squares.
Notes
The original version of these said to bake for 42 minutes, but that was much too long. Err on the side of underbaking for these, and chill before cutting.