Mistake: When cookies turn out flat, the bad guy is often butter that is too soft or even melted. This makes cookies spread. The other culprit is too little flour—don't hold back and make sure you master measuring. Finally, cookies will also flatten if placed and baked on hot cookie sheets. Keep it cool to start with.

How to Fix it: If too-soft butter was the culprit, try refrigerating cookie dough for 1 to 2 hours before baking. If too-little flour was the issue, try adding an additional 1 to 2 tablespoons of flour to the dough. Then, bake a test cookie. If neither were the mistake, it was probably a too-hot pan that caused the spreading. Cool your cookie sheet before baking another batch.

How to Avoid it: Allow your butter to come to room temperature—then cut it into pieces. The perfect temp should take about 30 minutes. Make sure you’re using the most accurate measuring method and the correct measuring tools. Cool your cookie sheet in between batches—allow the sheet to sit at room temperature for 5 to 10 minutes, then run under cold water and dry—this will cool the sheet without it warping.