Use the right brownie mix. Unfortunately, since all brownie mixes are not made equally, some mixes will leave you with super-thin cookies.
Don't over bake the cookies. They will get very tough if they over bake. Keep in mind they’re cook just a smidge on the cookie sheet before you move them to the wire rack to cool
Line the sheet pan with parchment paper. This will help prevent the cookies from burning on the bottom and make clean up a breeze.
Use a cookie scoop to help scoop out the dough. When you use a cookie scoop, all the cookies will be the same size to cook evenly. I like this medium cookie scoop.
Don't skip the chill time for the dough. If you don't let the cookies chill, they will spread out too much, and your cookies will be too thin.