Preheat oven to 350F (175C) with rack in middle position.

Grease a 13x9 inch baking pan and line with wax paper, then butter the paper.

Spread brownie batter evenly in the prepared pan.

In a small bowl, whisk together flour, baking soda, and salt.

In a large bowl, beat together melted butter and both sugars with an electric mixer until pale and fluffy (2-3 minutes).

Lightly beat 1 egg with a fork and add 1 3/4 tablespoons of it plus the remaining whole egg to the butter mixture, beating until creamy.

Beat in the vanilla extract.

Reduce mixer speed to low and mix in the flour mixture until just blended.

Stir in the chocolate chips.

Evenly spoon cookie dough over the brownie batter, spreading gently to form an even layer.

Bake until a wooden pick inserted in the center comes out with moist crumbs adhering, about 1 hour and 15 minutes.

Cool completely in the pan on a rack for about 2 hours.

Invert onto a cutting board, remove the paper, and cut into bars.