Preheat oven according directions on brownie mix .  Line an 8-inch square baking pan with, allowing a 2-inch overhang. Spray bottom of pan with non-stick spray.

Make batter according to direction on box for cake like brownies.  (This usually means just adding 3 eggs instead of 2)

Make filling: Stir together butter, confectioners’ sugar, peanut butter, salt, and vanilla in a bowl until smooth.

Pour one-third of batter into prepared pan; spread evenly with a rubber spatula. Drop dollops of half of the peanut butter filling (about 1 tablespoon each) on top of batter, spacing about 1 inch apart. I put 9 drops.

Drizzle remaining batter on top, and gently spread to fill pan. Drop dollops of remaining half of  filling on top. Gently swirl peanut butter filling into batter with a butter knife, running the knife lengthwise and crosswise through layers.  Be careful to not get the knife to the very bottom of the batter of you may cut the foil.

Bake according to brownie mix directions for 8 x 8 inch pan until a toothpick inserted into brownies comes out with a few crumbs but is not wet. Let cool slightly in pan, about 15 minutes. Lift out; let cool completely on a wire rack before cutting into squares.  Makes 9-12 brownies.