Preheat oven to 350°F (180°C). Spray an 8-inch (20 cm) square baking pan with cooking spray. In a medium bowl, using a fork, stir flour with cocoa, salt, and baking powder until no cocoa lumps remain. In a medium saucepan, melt butter over medium heat. Remove from heat and stir in sugar then egg, egg whites, and vanilla until combined. Stir in flour mixture just until blended.
Turn about 2/3 of the batter into prepared pan and smooth the top. Spoon dollops of remaining brownie batter over top. Do not smooth out. In a medium bowl, beat peanut butter with cream cheese until smooth. Beat in brown sugar then egg white until blended. Drop by spoonfuls in between chocolate batter spoonfuls. Using a thin bladed knife, draw the tip through the two batters to create a marbled effect.
Continue to do so until the batter is fairly level on top. Bake in preheated oven until the centre of brownie appears set and the sides just start to pull away from pan, about 28 to 30 minutes. Cool brownies in pan on a rack before cutting into squares.