Can be found, here: Charlotte Brontë · L’Ingratitude · LRB 8 March 2012. It is …
…a story written in French for her teacher Constantin Heger, has just been found by Brian Bracken at a museum in Charleroi.
Bonus! There is an audio version on the site as well, with the text read by Gillian Anderson.
Thanks to Maud Newton for pointing this out.