Sietze de Vries is a Dutch organist with an exceptional talent for improvisation. A quick search on youtube results in several videos where you can hear him improvise (https://www.youtube.com/results?search_query=de+vries+improvisation). A speciality of him seems to be to improvse choral partitas in the style of Northern German Baroque. And they are really good. If you were to write them out, you would have a piece of music that would do well as a genuine composition.
To transcribe music by ear is quite difficult to do. The upper voice if perhaps the easiest to hear and the bass voice shold be doable as well. However, what happens between those two outer voices? To precisely hear what all four voices do is another talent that I think not many people possess.
Joachim Scheufele-Leidig is one of those people. He listens to the improvisations of Sietse de Vries and writes some of them out. An example is de Vries' improvised choral partita on "Von Gott will ich nicht lassen", improvsed in 2015 in St. Jakob i in Hamburg. The recording is available on youtube:It consist of and introduction and 8 variations on the choral melody. Scheufele-Leidig transcribed the second of these variations, a chorale prelude with colourated cantus firmus in Buxtehude-style (starts at 4:38 in the youtube video).
The result is music that is a joy to play. It is unbelievable someone could improvise this on the spot. I had to practise several hours to be able to play this piece. And it is intruiging that someone can listen to it and decide to write it out. I am glad de Vries improvised this piece and that Scheufele-Leidig decided to notate it on paper.
As a tribute to both gentlemen I played it twice: once on the sample set of the Hinsz organ in the Dorpskerk in Midwolda and once on the sample set of the Schnittger organ in the St. Martini in Groningen. The latter is the church where Sietze de Vries is organist. Both sample sets were produced by Sonus Paradisi (see here for Midwolda and here for Groningen)
Scheufele-Leidig's own performance ánd the score can be found here