Who composed the Fantaisie-Impromptu in C♯ minor?

Frédéric Chopin's Fantaisie-Impromptu in C♯ minor Op. posth. 66 is a solo piano composition. It was composed in 1834 but published posthumously in 1855 despite Chopin's instruction that none of his unpublished manuscripts be published. The mystery of him not allowing it to be published may have been solved in 1960 when pianist Arthur Rubinstein acquired the "Album of the Baroness d'Este" which had been sold at auction in Paris. This album contained a manuscript of the Fantaisie-Impromptu in Chopin's own hand, dated 1835, stating on the title page in French "Composed for the Baroness d'Este by Frédéric Chopin".The Fantaisie-Impromptu is one of Chopin's most frequently performed and popular compositions.

The C♯ minor Moonlight sonata of Beethoven, particularly the third movement, is held to have been the inspiration for Frédéric Chopin's Fantaisie-Impromptu, and that the Fantaisie-Impromptu was actually a tribute to Beethoven.

Ernst Oster (was a pianist, musicologist, and music theorist) wrote, "Chopin understood Beethoven to a degree that no one who has written on the C♯ minor Sonata or the Fantaisie-Impromptu has ever understood him. The Fantaisie-Impromptu is perhaps the only instance where one genius discloses to us—if only by means of a composition of his own—what he actually hears in the work of another genius."