Preheat the oven to 350℉. Spray a 9x9-inch baking dish with nonstick spray.
MAKE THE COOKIE LAYER:
In a large bowl, use an electric mixer to combine the butter, sugars and vanilla until creamy. Add the egg and continue to mix for about 1 minute. Add the flour, baking soda and salt; beat to combine. Stir in the chocolate chips. Spread the dough evenly over the bottom of the prepared pan.
MAKE THE BROWNIE LAYER:
In a large bowl, use an electric mixer to combine the sugar, butter and vanilla until creamy. Add the eggs and beat to combine. Add the flour, cocoa powder, baking powder and salt; beat until combined. Pour the brownie batter over the cookie dough and spread to cover completely.
Bake for 30 minutes. Cool before cutting into bars. The center of the bars may sink slightly. I like to cut off the edges and then cut the rest into bars.
Notes
Do not use anything smaller than a 9x9-inch baking dish. If you have an 11x7-inch baking dish, you can spread the batter thinner in that size to create bars that won't be as thick.