Preheat oven to 350 and line an 9x9 baking pan.
Melt the butter and chocolate over boiling water or in the microwave, stir every 15 seconds.
Once the chocolate and butter are melted, add in granulated sugar and whisk well.
Add in eggs one after another, mix well after each addition.
Mix in flour and stir well so no lumps are left.
To make peanut butter swirl, mix peanut butter, powdered sugar and melted butter in a small bowl.
Pour the brownie batter into the baking sheet.
Using a spoon, pour the peanut butter mixture on the batter and swirl using a knife.
Bake in the oven for 20-25 minutes until the edges are set but the middle is still jiggly.
Cool completely and then cut into 16 pieces.