Prep the pan. Heat the oven to 350°F (175°C). Line a square pan with parchment, leaving overhang on two sides. Lightly grease the parchment.

Make the brownie batter. In a heatproof bowl, melt the butter and chocolate together over a gentle simmer or in the microwave in short bursts. Stir smooth. Whisk in granulated and brown sugars. Whisk in eggs and vanilla until glossy. Sift in flour, cocoa, and salt. Fold just until combined.

Spread and mint. Pour the brownie batter into the pan and smooth. Sprinkle about half of the chopped Andes mints evenly over the batter. Press lightly so they settle in.

Make the cookie dough. In a separate bowl, beat the softened butter with brown and granulated sugars until fluffy, 1–2 minutes. Beat in the egg and vanilla. Whisk flour, baking soda, and salt in a small bowl, then add to the wet ingredients. Mix just until no dry spots remain. Fold in chocolate chips and the remaining chopped Andes mints.

Layer the cookie dough. Using a spoon or small scoop, dollop the cookie dough over the brownie layer in small mounds. Gently spread to mostly cover, leaving a few gaps so steam can escape. Do not press hard.

Bake. Bake 30–36 minutes for a 9-inch pan (34–40 for 8-inch), until the cookie top is golden and set at the edges. A tester inserted in the center should come out with a few moist crumbs but not wet batter.

Cool completely. Let the brookies cool in the pan on a rack for at least 1 hour, preferably 2. This sets the layers and makes cutting clean squares much easier.

Slice and serve. Use the parchment overhang to lift out. Cut into 12–16 squares. For neat edges, wipe the knife between cuts.