Start with what went wrong. Every answer here is checked against 85 recipes rather than repeated.
Most troubleshooting advice is a list of plausible causes in no particular order, and the lists disagree with each other. We compared every published recipe we could resolve and read 7,124 reviews from cooks who changed one thing, so we can rank causes by evidence and say plainly which ones we cannot test.
Most cookie problems are not formula problems. These three account for more bad batches than every ingredient argument combined.
A cup scooped from the bag can hold a third more than a cup spooned in and levelled.
This is the single most common cause of both flat and cakey cookies, in opposite directions, and it is the one thing our data can never see. Recipes record what they asked for, not what went in. If you change one habit, change this one.
What to do: 120 g per cup of plain flour, on a scale.
Cold, softened, melted and browned butter are four different ingredients, not four temperatures of one.
Butter is roughly a sixth water. Whether that water is locked in fat crystals or already free changes how much gluten forms before the tray ever reaches the oven. A recipe written for creamed butter will not behave if you melt it, and the reverse is just as true.
What to do: match the recipe's method before blaming its ratios.
Cookie size is a formula variable, not a serving choice.
Mass decides how far the edge dries out before the centre sets. A large cookie can be crisp at the rim and molten in the middle from exactly the dough that bakes uniformly at half the size, which means half of what looks like a recipe fault is a scoop fault.
What to do: weigh one portion, then match the rest.
Pick the sentence closest to what came out of your oven. Where we have a full page, the short answer links to it; where we do not, the short answer is the whole answer and we say so.
A cookie spreads for as long as its fat is liquid and stops the moment its structure sets. Everything that goes wrong with a cookie's shape is that one race, finishing too early or too late.
Which is why the two most-searched cookie problems have the same levers with the signs reversed, and why fixing one by overcorrecting is the usual route to the other. Nobody covering both seems to link them, so readers arrive at each page as though it were an unrelated fault.
We can say which way a change pushes and which change pushes hardest. We cannot tell you how wide, how tall or how dark the result will be, and we will not guess.
Some popular explanations have no comparison group in the published record at all. We say which, rather than quietly leaving them off the list or repeating them as fact.
Mis-measured flour probably causes more bad cookies than everything else combined, and no recipe records what actually went into the bowl. We flag it first and admit we cannot rank it.