To make cookies using a KitchenAid mixer, the best attachment to use is the flat beater. The flat beater is a multi-purpose attachment that can be used for mixing cakes, cookies, frostings, and other batters. It is designed to mix ingredients thoroughly and can do everything from basic stirring to creaming butter against the bowl. The flat beater is also recommended for creaming butter and sugar and adding in eggs when making cookie dough. While other attachments like the flex edge beater can also be used for mixing cookies, the flat beater is the most commonly recommended attachment for this task.