Making the Dough
In tests, we’ve found that the most common way of measuring dry ingredients - spooning them into the measuring cup—is also the least accurate. Since even the slightest variation in an amount can have a direct effect on your cookie (a tiny bit too much flour, for example, and the cookie will be dry; too little and the cookie will bake up flat), it’s important to measure precisely.
Use Butter at Optimal Temperature
Whether softened or melted, proper butter temperature is as critical in a simple sugar cookie as it is in the fanciest cake.
Properly softened butter (65 to 67 degrees, or roughly room temperature) allows air to be pumped into the butter for tender texture in the final cookie. Two good cues: The butter should give slightly when pressed but still hold its shape, and it should bend without cracking or breaking.
When a recipe calls for melted butter, make sure it’s lukewarm (85 to 90 degrees) before adding it to the dough. Butter that’s too warm can cook the dough (or the eggs in it) and cause clumps.