1. FatButter produces a better flavour and a more uniform cookie than margarine. Its low melting point makes it start to spread in the oven right when it's supposed to. Make sure it's at room temperature in order to properly beat it with the sugars, which will aerate (incorporate air into) the dough. Melt your butter first (brown it, even) for a denser, chewier cookie. More butter will result in more spread, and a thinner, crisper cookie.
2. SugarSugar is what makes cookies brown in the oven; a higher proportion of sugar – white in particular – will result in a crispier cookie.
3. EggsRecipes assume you're using large-sized eggs. Eggs bind dough together and act as a leavening agent, producing puffed, cakey cookies. Less egg will result in thinner cookies.
4. FlourAll-purpose is perfect, but bread flour, which is higher in protein, is sometimes used to produce cookies that spread less.
5. ChocolateAs with any baked goods, the better quality chocolate you use, the better the cookie. Dark and semi-sweet are best; their flavour comes through more strongly than mellower milk chocolate. Swapping chopped chocolate for more uniform chocolate chips. This will allow a wider distribution, with a range from wee bits to big pockets of melting chocolate throughout your cookie.
6. DoughSome people refrigerate their finished dough to solidify the fat, slowing its spread in the oven; others keep it there for a few days to age the dough, giving it a denser texture and more well-developed flavour.
7. BakingA heavy, light-coloured baking sheet will bake cookies evenly without burning their bottoms. A cookie scoop is a worthwhile investment – same-sized cookies will cook in the same length of time. Bake one sheet at a time, unless you have a convection oven. And remove them from the oven when they're golden around the edges but still soft in the middle; they'll firm up as they cool.