I have been achieving amazing burgers thanks to Kenji's tips (http://aht.seriouseats.com/archives/2010/03/the-burger-labs-top-ten-tips-for-better-burgers.html) as well as some other articles.
But yesterday I went to a new NY restaurant (Kosher) and had a really good burger. What I loved from it was the crunchiness on the crust, not too crunchy but just a little bit, which made it a really great eating experience.
I am wondering how to achieve this crunchiness at home.
I usually use a cast-iron pan, I buy ground meat and apply the tips quoted before. I go with the 15-second flip method and keep the burgers cooking for a total of 4 minutes.
What should I add to achieve the crunchiness?