For the Brownie Layer:

1. Preheat the oven to 350°F. Generously grease a 9 x 13-inch pan, or an 8 x 8-inch square pan if you want thicker layers.
2. In the bowl of a stand mixer fitted with the paddle attachment, mix together the sugar, flour, cocoa powder, and salt on low speed. Add the oil, eggs, and vanilla. Beat on medium speed until a thick batter forms.
3. Pour the mixture into the prepared pan. Bake for 35-40 minutes (an additional 15-20 minutes for a square pan) until set. Remove from oven and cool completely.

For the Cookie Dough Layer:

1. In the clean bowl of a stand mixer fitted with the paddle attachment, cream the butter with the granulated sugar, brown sugar, and salt. Add milk and vanilla, and mix until combined. Add the flour and mix until a thick dough forms. Mix in the chocolate chips.
2. Spread the dough over the cooled brownies. Sprinkle the top with extra chocolate chips.
3. Chill in the refrigerator until the cookie dough layer sets, about 30 minutes. Cut into 12-16 even squares and serve.