What to check when drop cookies spread too far
Bake a small trial and compare dough temperature, pan preparation and ingredient weights one change at a time.
First decide whether anything went wrong. A thin, crisp cookie is supposed to spread. If the tested recipe shows plump rounds but your first tray merged, set the rest of the dough aside and compare your process with the instructions. Note the butter state, flour weight, oven setting and the way you prepared the baking sheet.
One quick trial uses two equal portions. Put them on a cool sheet prepared as the recipe specifies, leaving ample space, and bake in the fully preheated oven. If the first tray was warm when you loaded it, this trial removes that variable. If the dough is unusually soft and the recipe allows chilling, chill another pair and compare them at the same portion size.
If both trials still flatten, check what you measured. A scoop of flour or an egg substantially different from the recipe amount changes the dough itself. Write down the weights for the next batch. Adding handfuls of flour to an already mixed bowl may thicken it, but it also changes the texture you were trying to reproduce.
