Use fresh unsalted butter for best results. Let it stand at room temperature until it’s malleable, but not too soft: 30 to 60 minutes depending on the temperature of the room. It should give slightly when pressed but still maintain its shape. The ideal is 67°F — the temperature at which butter will best hold air and be stable. Butter that’s too soft will make a sticky dough that’s harder to work with, provides less leavening, and spreads too much during baking. To speed up the softening time, cut butter into tablespoon-size pieces.
Make sure your oven is calibrated and, for best results, heat it for at least 20 minutes before baking.