Heat oven to 350℉. Prepare a 9x13 baking dish by spraying with cooking spray or lining with parchment paper for easy removal.
In a large mixing bowl, whisk together the melted butter and sugar until combined.
1 cup (2 sticks) salted butter, 2 cups granulated sugar
Add eggs, one at a time, mixing the egg into the batter with the whisk before adding the next egg.
4 large eggs
Add cocoa powder and vanilla extract. Stir to combine.
6 tablespoons unsweetened cocoa powder, 2 teaspoons vanilla extract
Add the flour and stir together until no flour streaks remain in the batter.
2 cups all-purpose flour
Add the chopped cookies into the brownie batter and stir to combine.
1 cup crushed Keebler Grasshopper Cookies
Evenly spread out the brownie batter into the pan. The batter is thick so it will take a few minutes to spread out. It helps if you dollop spoonfuls of it all over the pan and then spread it out. If you spray the back of a spoon with cooking spray you can use that to help spread it out too.
Bake for 25-32 minutes. The brownies will look slightly puffy and pulled away from the edges of the pan. A toothpick inserted in the middle should come out clean with no wet batter.
While the brownies are baking unwrap all the Andes Mints.
2 packages (4.67 oz each) Andes Mint Chocolates
Right when the brownies come out of the oven, put the unwrapped Andes Mints in rows on top of the hot brownies. Let it sit for about 10 minutes and then take a spoon and spread out the melted mint chocolate over top the brownies. *OPTIONAL STEP : After you have spread the frosting out, you can chop up some additional Andes mints and sprinkle them over top the frosting. This just gives the brownies an extra special look with the chopped pieces on top (similar to what is pictured in the photos). Let the brownies cool completely, inside the pan, at room temperature. As the brownies cool, the mint frosting will firm up and harden; it takes about 2 hours.
Cut into squares and serve at room temperature. Enjoy!