They are sometimes interchangeable, but often not. In this specific sentence, there is a very subtle difference:
- I am writing down the recipe: אני רושמת את המתכון
- I am writing the recipe: אני כותבת את המתכון
In other words, רושמת here clarifies that she is in fact copying the recipe from somewhere else, while כותבת doesn't imply that, and the connotation leans more toward original writing.
Sometimes I google words with images. When I googled רשם, which is probably the noun that the verb came from, I saw pics of people meeting each other, shaking hands, going on dates. Then I went to an online Hebrew-English dictionary, and the noun means "impression." Pens and pencils can also be used to make impressions of something, so that's how I keep the two different verbs distinct in my head. I have heard that most abstract ideas in Hebrew are based in the concrete, and this would be a good example of that.
this idea is actually what fluent forever app and website are based on, they have most frequently used words list and the definition fit 600 words in the languages and to can buy illustrations to use on Anki or flashcards, so you don't see the English translation, the app is Kickstarter most funded app ever, it was on a cover of magazines... https://fluent-forever.com it's not out yet but he shows to how to use the images until it is.