Place peanut butter in a microwave safe bowl and heat for 20 to 30 seconds or until pourable.
Line a metal 8-inch square pan with parchment paper, leaving an overhang.
Spread peanut butter into an even layer in pan and freeze for 1 hour or until solid. Use parchment paper to remove from pan and place back in freezer.
Preheat oven to 350 degrees. Line the square pan again with parchment paper, leaving an overhang.
In large microwave safe bowl, combine butter and chocolate. Microwave in 30-second bursts, stirring frequently, until melted and smooth. Add sugar to mixture and whisk vigorously until combined.
Allow to cool until barely warm. Add in eggs, yolk and vanilla and whisk for 1 minute until well combined.
Using a rubber spatula, stir in flour, cocoa powder and salt until just combined.
Pour half of batter into prepared pan and smooth out.
Carefully place frozen peanut butter sheet over batter. Pour remaining batter on top spreading to cover completely.
Bake for about 30 minutes or until cooked through but slightly gooey in center. Let cool in pan for 30 minutes.
Remove from pan and allow to cool another 30 minutes before serving.