Why did my cookies turn out flat?

Many factors contribute to a flat cookie. The biggest one tends to be the temperature of the butter when the ingredients are mixed together. If you want a taller, more dense cookie, it is recommended you use cold butter. The best way to do this is to take your sticks of butter and cut it into cubes or slices. Then your mixer will have an easier time incorporating the butter into the sugars and other ingredients. Melted butter will definitely result in a thinner more flat cookie.