For brownie layer:.
Position rack in center of oven and preheat to 325°F Spray 9-inch square metal baking pan with 2-inch-high sides with nonstick spray; line bottom with parchment paper.
Stir chocolate and butter in medium saucepan over low heat until smooth; cool 10 minutes. Using electric mixer, beat sugar, eggs, and vanilla in medium bowl until very fluffy, about 3 minutes. Beat in chocolate mixture on low speed, then beat in flour and salt. Spread batter in pan.
For cheesecake layer:.
Using electric mixer, beat cream cheese, peanut butter, and vanilla in medium bowl until smooth. Gradually add sugar, beating until fluffy. Beat in eggs 1 at a time. Beat in cream, then flour. Transfer 1/3 cup cheesecake batter to small bowl and reserve. Spread remaining cheesecake batter over brownie layer.
Place chocolate in small microwave-safe bowl. Microwave on low power in 10-second intervals until chocolate begins to melt, then stir until smooth. Mix chocolate into reserved 1/3 cup cheesecake batter. Drop chocolate batter by heaping teaspoonfuls atop cheesecake layer. Using chopstick or wooden skewer, swirl chocolate batter in figure-eight pattern through white cheesecake layer.
Bake brownies until edges of cheesecake layer are puffed and center is set, about 36 minutes. Cool brownies in pan on rack. Cover; chill at least 1 hour.
Cut brownies into 25 squares. Using narrow spatula, transfer brownies to platter. Serve cold.