For the brownies

1. Preheat oven to 350°F. Line an 8×8-inch pan with parchment paper or foil and grease lightly.
2. Combine the melted butter, sugars, and vanilla in a large bowl. Beat in the eggs, one at a time, mixing well after each, until thoroughly blended.
3. In a separate bowl, combine the flour, cocoa powder, and salt. Gradually stir flour mixture into the egg mixture until blended. Spread the batter evenly into the prepared baking dish.
4. Bake in preheated oven until an inserted toothpick comes out clean, about 30 minutes. Remove, and cool pan on wire rack before adding cookie dough on top.

For the cookie dough layer

1. In a medium bowl, mix together the brown sugar, white sugar, and butter until smooth. Whisk in vanilla and milk. Mix in the flour until well blended. Stir in chocolate chips.
2. Spread cookie dough in an even layer on top of cooled brownies. Refrigerate until firmed up, about 1 hour, before cutting into squares.