Solution

As my chess quizzes require not only a single move to be found but a whole move order, it isn't always easy to declare a winner (as often more than one participant contributes valuable ideas). In this case I decided to declare Hive account@holbein81 and Hive account@greatwolf both as winner, because both had the correct ideas.

The complete solution is as follows:

1. Bb5+! Nxc2 (1. ... Nc6 2. Bxc6 and white wins as well.)
2. Rxc2+ Bc5
3. Rxc5+ Qc7
4. Rxc7+ Kb8
5. Rc8++! Kxc8
6. Rc1#

Congrats to the winners!