1. Notify boss your intent to join another company in exactly 2 weeks. No need to explain or apologize or thank anybody. No need to make transition longer either unless you don't feel eager to start the new job, which brings up the question of why bother.

2. Tell boss you'd like to focus on a smooth and orderly handover during this 2 weeks. Invite her to share any ideas she may have to accomplish this goal

3. Conclude with the sentence: I am eager to leave on the most positive note and I am sure you and others in the group would like the same.

Make your letter brief. No need to mention anything about your new gig: title, responsibility, salary, or even the name of the company.

Your message should be absolutely clear: I have made up my mind to start with another opportunity and let's decide together what's the best way to make use of the following 2 weeks.

Don't feel sorry for the boss or colleagues. Everybody should do what's in his/her best interest. Freedom to choose, like Milton Friedman would say, is the essence of true capitalism.