Solution.

Congrats to @iceblue for finding the key move of today's chess problem, and thus being the winner of this round! Congrats also to @rodrook for the solution of a side variation! :)

Here now the full solution of the problem:

1. ... a3

Now we have:

a)

2. Kc3 Bb4+!
3. Kxb4 a2 And black wins.

b)

2. Bc7 (Idea: 2. ... a2 3. Be5) 2. ... Bd4!!

Now white can take the black bishop with pawn, knight or king, but in every case he cannot stop the black passed pawn anymore.

If you enjoyed the riddle, show it to other chess players and ask them if they can solve it! :)