Book about a russian chocolatier

I am trying to find a book from my childhood. I must have read this in 2017-2019. This is a child/ young adult book, but was a little dark to be categorised as such i would say. This book was illustrated by Chris Riddell and might be written by an author called Christopher something. A boy is orphaned and sent to his great uncle, who is one of the 12 great chocolatiers in the region/city. A serial killer is targeting these chocolatiers and killing them one by one. His great uncle is very cold and distant towards his nephew and does not display much familial affection for him, presumably because he has not met him before. He is a hardened old man. I think the book is set in Russia/Germany, but is not modern day, possibly 1890s anywhere up to 1950s?? The boy and his great uncle prepare for the cities legendary chocolate festival, for which they produce elaborate sculptures from chocolate. Please someone let me know if you remember this book or if you have any extra details you might remember!!