If your spoken English is good, you can call first. This is more interactive and can leave a deep impression on the other party.
Pay attention to grammar when writing letters, and don't write too complicated sentences, lest you make mistakes and give people the impression that your German is not good enough.
At the beginning of the letter, you should mention the time, task and place of the last interview, indicating that we had a pleasant meeting last time and hope to have more opportunities to learn more about it.
Ask the other person what he thought of the last interview and so on.
So that the other party will understand what you mean when they receive the letter.