Heat the oven to 350°F (175°C). Line a 9 x 13-inch baking pan with parchment paper, leaving enough overhang to lift out the brownies, or lightly grease the pan.

Whisk the melted butter and granulated sugar together in a large bowl. Add the eggs and vanilla extract, then mix until the batter is smooth and evenly combined.

In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, and salt. Gradually stir the dry mixture into the butter mixture just until no dry streaks remain; do not overmix.

Spread the batter evenly in the prepared 9 x 13-inch pan. Bake for 25–30 minutes, until the center is set but still slightly soft and a toothpick comes out with moist crumbs rather than wet batter.

Immediately arrange the 28 unwrapped Andes Creme de Menthe Thins over the hot brownies. Let them stand for 2–5 minutes to soften, then spread the melted mint chocolate into an even layer.

Cool the brownies completely before cutting. For especially neat slices, refrigerate them briefly after the topping has set, then lift them from the pan and cut into about 24 pieces.