Preheat oven to 180 degrees. Line and grease a 9x13 inch pan.
First prepare the cookie layer. Beat the butter, oil, sugars, vanilla and egg till well combined.
Sift in the flour + baking soda and salt into the beaten mixture and fold till not dry bits are present.
Spread evenly on the prepared pan and set aside.
For making the brownie batter, whisk together the sugar, butter, vanilla and eggs till the sugar is all melted and combined.
Fold in the dry ingredients till done. Spread over the cookie base.
Bake for 25 to 30 minutes, till a skewer inserted comes out clean.
Allow to cool in pan for 10 minutes. Slowly pull out from the pan. Initially the cookie batter will be soft, so handle with care. Cool on rack completely. The cookie batter will harden in due time.
Cut into bars and serve!
Notes
You can fold in some chocolate chips into both the batters to make it more decadent. Skip the salt if using salted butter.