These cookies are spreading into 3 mm thin discs. How do I fix this? I already chilled the dough overnight.

Based on what I know of cookie science I would suspect one of the bottom three factors in the culprit:

1) Too little flour
2) Too much butter
3) Your oven is running cold

All of these can contribute to cookies spreading in the oven to a thinker consistency then normal.

Hope that helps!

Hi Lianna! Hm, that's puzzling. Any chance you flattened the cookie dough scoops before baking? If not, I wonder if the flour measurement ended up short somehow? Hope they're still tasty—even though thinner than intended.