Preheat oven to 350°F and grease a 9x9-inch baking pan.
Mix flour, cocoa, salt, and baking powder in a bowl.
Cream butter and sugar until fluffy, then add eggs and vanilla.
Fold in dry ingredients and chocolate chips.
Pour half the batter into the pan and spoon peanut butter on top.
Add remaining batter and swirl peanut butter with a knife.
Bake 25–30 mins until a few crumbs stick to a toothpick.
Cool 15 mins, transfer to rack, and cut into squares.