• Preheat oven to 325°F. Grease an 8 or 9-inch square baking pan and line with parchment paper, letting the sides overhang.

To make the filling:

• Combine the melted butter, powdered sugar, peanut butter, and vanilla extract and stir until smooth. Set aside.

To make the brownies:

• In a saucepan over medium heat, melt the butter and chocolate. Stir until smooth and remove from heat as soon as all of the chocolate is melted. Meanwhile, combine the flour, baking powder, and salt in a bowl.
• Add the sugar to the saucepan and mix until incorporated. Add the eggs one at a time, mixing after each. Stir in the vanilla. Add the dry ingredients and mix just until incorporated.
• Spread batter into the prepared pan.
• Scoop the filling by the rounded tablespoon on top of the brownie batter, leaving about 1 inch between each scoop. Run a knife through the batter and filling, touching the bottom, to swirl the two together.
• Bake until a toothpick inserted into the center comes out with a few dry crumbs, about 35-45 minutes (depending on what size pan you use). Let cool completely in the pan before slicing and serving.
• Brownies may be stored at room temperature or in the fridge for up to 3 days.