Preheat oven to 350F. Line a 9x9-inch pan with foil and spray with cooking spray.
Place baking chocolate and butter in a large, microwave safe bowl. Heat on HIGH power for 2-3 minutes, stirring every 30 seconds, until chocolate is smooth. Stir in sugar. Add eggs, vanilla, and cocoa and stir well. Add flour and stir carefully. Spread batter in prepared pan.
Stir together peanut butter, melted butter, powdered sugar, and vanilla. Drop spoonfuls on top of the brownies then swirl with a butter knife or the back of a wooden spoon.
Bake about 28-35 minutes, until the center seems set. Let cool before cutting.
Notes
These peanut butter brownies are rich, fudgy, and perfect for any chocolate and peanut butter lover!