Took me 30 seconds to spot it. I was surprised by how long it took for me to see it. Then again I was playing the game!

No black can counter with Qa5!

*clap,clap* nice 

or f6

Took me like a minute :p  I thought I checked for it, so had to wait till I came back and looked again.

I also saw though that Qa5 isn't a counter because white has Nb5 (double attack knight and c7).  If knight moves then Nxc7 threatens both takes on d6 and pawn to e7.

I discarded b4 Nd5 because BxN... but there are two knights that can go to d5.  So Qa5 also fails to the move b4.