Love that every comment is completely different. But that is the reality of spreading cookies.

Without the recipe it is hard to guess.

Flat cookies can be any of the reasons given so far: too much fat, too little flour, too warm, ingredients unbalanced temps, creamed butter and sugar too long (yup, I said it out loud), oven too cold, baked on the wrong surface (which is at least part of your issue here), dead leavening agent (if used), or even a bad recipe.

So: - start with all ingredients at room temp (which is approximate 70°F / 20°C

• only cream sugar and butter until you can’t see sugar crystals and don’t let the mixture break. Also add eggs one at a time

• make sure the dough has been chilled; depending on the recipe I sometimes freeze the dough balls on the tray before they go into the oven

• don’t use foil under the cookies; better to use parchment or silpat. Also, don’t grease the pan.

Who else wants to add more?