MOST cookie recipes require the butter to be at room temperature:
http://whatscookingamerica.net/Cookie/CookieTips.htm
"Butter: Make sure your butter is at room temperature, otherwise it won't cream properly with the sugar. The terms "room temperature," "softened" and "soft" mean different things. The temperature of the butter an make a difference in the recipe. Most cookie dough recipes depend on the emulsion that occurs when you cream butter and sugar together. This emulsion will not happen if the butter is too hot or too cold.
Room Temperature Butter: It should be pliable enough that your finger can leave a mark in it, without being soft and greasy. Set the butter out at least one (1) hour in advance.
Softened Butter: Will feel a little warmer to the touch, and it will be much easier to leave a deep indentation, but it should still be firm enough to pick up without falling apart.
Soft Butter: Will be too soft to pick up.
Do not try to microwave your butter as it will just end up too soft.
TIP: If you don't have an hour's lead time, increase the surface area by cutting the butter into small pieces or shredding it on the large holes of a grater. It will then come up to temperature in approximately 10 minutes.
Unsalted butter is generally recommended because some salted butters have more sodium than others.
TIP: If you use salted butter, only use 1/2 the amount called for in the recipe. Don't skip the salt, as salt brings out flavors and balances the sweetness in a recipe."
My wife uses the "cold butter cut into small cubes" technique for making puff pastry...fabulously flaky and tender.
BTW, if you like brownies...this recipe makes the best brownies I've ever had...
http://www.nickmalgieri.com/blog/w10_brownies.html