Over mixing the batter will cause your cookies to be tough rather than tender and soft.Only mix the dry and wet ingredients together until most of the flour is hydrated.
It's really important to chill this cookie batter for at least three hours and up to three days!This will improve the flavor and texture of the cookies.
For the best balance of flavor, I add about a half cup of sourdough starter to this cookie dough!It adds a slight tang to the cookies that makes them oh so good!