Preheat the oven to 325 degrees F. Line a 9-inch square, 2-inch-deep baking dish with foil, leaving an overhang; butter the foil. Make the brownie layer: Beat the butter, granulated sugar and brown sugar in a bowl with a mixer on medium speed until creamy, 4 minutes. Add the eggs and vanilla; mix until combined. Reduce the mixer speed to low; beat in the flour, cocoa powder and salt. Spread in the prepared pan.
Make the cookie layer: Beat the butter, brown sugar and granulated sugar in a clean bowl with a mixer on medium speed until creamy, 4 minutes. Add the egg and vanilla and mix until combined. Reduce the mixer speed to low; beat in the flour, baking soda and salt. Stir in the chocolate chips. Gently spoon and spread the cookie dough on top of the brownie batter. Bake until a toothpick inserted into the center comes out clean, 35 to 40 minutes. Transfer to a rack and let cool completely in the pan.
Make the cereal-treat layer: Melt the butter in a large saucepan over medium heat. Add the marshmallows; stir until melted. Remove from the heat and stir in the cereal. Let cool slightly. Butter your hands, then press the cereal mixture on top of the cookie layer. Refrigerate until set, about 30 minutes. Cut into squares.