Preheat your oven to 350°F (175°C) to ensure even baking.
Line your 9x9-inch baking pan with parchment paper, leaving some overhang.
In a large mixing bowl, combine the softened butter, brown sugar, and white sugar. Beat until light and fluffy.
Add the eggs and vanilla to the mixture, beating well until fully incorporated.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually mix into the wet ingredients until just combined.
Fold in the chocolate chips evenly throughout the batter.
Spoon half of the batter into the prepared baking pan and drop spoonfuls of peanut butter on top. Swirl gently with a knife.
Spread the remaining batter over the peanut butter layer and swirl again if desired.
Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
Allow to cool for 10 minutes before lifting out and cooling completely on a wire rack. Cut into squares to serve.