Heat oven to 275 degrees.
Spray a 9-inch square pan with baking spray. Set aside.
In a medium bowl, combine 7 Tablespoons of the butter with the peanut butter.
Microwave in 30 second intervals, stirring after each, until melted and creamy. Set aside.
Place chocolate and remaining butter in a medium bowl and microwave in 30 second intervals, stirring after each, until melted .
Stir until smooth and combined completely. Set aside.
In the bowl of a stand mixer, combine eggs, sugar, and vanilla.
Beat at high speed for 10 minutes.
Reduce speed to low and stir in flour.
Pour half of batter into the peanut butter mixture and half into the chocolate mixture.
Stir each until thoroughly combined.
Spread half of peanut butter mixture in bottom of prepared pan.
Top with all of chocolate mixture.
Spoon remaining peanut butter mixture in randomly spaced dollops on top of chocolate mixture.
Use a knife (or something similar) to swirl the peanut butter with the chocolate.
Bake until puffy and cooked through, about 1 hour and 10 minutes.