Preheat oven to 350°. Lightly butter an 8-inch square baking pan. Line with parchment, leaving about a 2-inch overhang on two sides. Lightly butter parchment.
Whisk together butter and both sugars in a large bowl. Whisk in eggs and vanilla. Gradually add flour and salt, mixing just until moistened.
Divide batter in half, and transfer one half to a separate bowl. Stir in cocoa powder and chocolate chips until combined.
Place chocolate batter in baking pan. Spread evenly and smooth top. Spread plain batter on top of chocolate batter. Smooth top.
Bake 25-30 minutes, or until top is golden brown and a toothpick inserted in the center comes out clean. Cool completely in pan on wire rack.
Makes 24 bars.