Preheat oven to 350°F. Line a 9x13” pan with foil and spray with cooking spray.
Mix brownies according to box directions or stir all ingredients called for if using a homemade mix. Spread in the bottom of the prepared pan.
Cream butter and both sugars until smooth in the bowl of a stand mixer fitted with the paddle attachment (you can also use a hand mixer). Beat in egg, vanilla, baking soda, and salt until smooth. Mix in flour, then stir in oats and butterscotch chips.
Drop spoonfuls of the cookie mixture evenly over the top of the brownie batter. Press down lightly with your hands. (You don’t want to mix the two layers, just flatten the cookie dough a little.) The cookie dough will not completely cover the brownies but get it as close to the edges as you can.
Bake for 25-30 minutes or until the cookies are golden brown. A toothpick is not a good test for these. The brownies cook but stay very fudgy!
Cool completely before slicing. Store in an airtight container for up to 3 days for freeze for up to one month.