Preheat the oven to 350℉. Line a 9x13-inch pan with foil or parchment paper, leaving a 2-inch overhang on two sides. Spray with nonstick spray.
PREPARE THE BROWNIE BATTER:
In a large saucepan over low heat, combine the butter and chocolate. Stir occasionally, until melted. Remove from heat and let cool slightly. Add the sugar and eggs to the chocolate mixture and stir with a whisk. Add the flour, cocoa powder and salt, and whisk until combined.
PREPARE THE BLONDIE BATTER:
In a large bowl, use a whisk to combine the melted butter, brown sugar, eggs and vanilla. Stir in the flour, baking soda and salt. Then stir in the butterscotch chips.
ASSEMBLE AND BAKE:
Use a rubber spatula to spread the brownie batter evenly in the pan. Press a single layer of cookies into the top of the brownie batter. Then carefully spoon the blondie batter on top and spread to cover evenly.
Bake until a toothpick inserted into the center of the bars comes out clean, 50 to 55 minutes. Remove the pan from the oven and let the bars cool completely.
Lift the bars out of the pan using the overhanging foil/parchment, then peel it off. Cut into squares.