So I've been at my first professional job as an engineer for one year.  Ironically, on the one-year anniversary of my hire, I received a much better offer from a different company.   The ways in which it is better are myriad; better pay, better experience for my resume, better benefits, better location.  It's indisputably better and my current employer can't match it.

I've learned a lot in the past year about professional behavior as well as about engineering.  I am very genuinely grateful to my current employer for that crucial first year of experience, as well as the mentoring that has occurred for me here.   I've done good work.  I've been worth my paycheck.  I'm proud of what I've accomplished and my boss and peers have made sure I know I'm a valued part of the team.   I've made a lot of casual friends.

The problem is that I don't know how to tell them I'm leaving without seeming like a jerk.  I know I'm supposed to be calm and businesslike, put it in writing, be prepared to either leave immediately or work diligently those last two weeks, thank my boss, keep up with my contacts, etc.  Now is a very bad time for me to leave and them to have to train someone new.  I know that is not really my problem, but I feel some responsibility to my projects.

Any suggestions/advice on how, exactly, to break the news politely and professionally?