The chord known as the Chopin Chord is a form of dominant 7th chord, but as well as the dominant 7th there is also a flattened 6th note or 13th note. So, for example, in the key of
C minor, the chord looks like this:
Here is an example from Chopin's Mazurka op.30 no.2 bar 25-6
and another from the Sonata Op.35 no.1. Where the previous two examples were in a minor key, there was a tendency for the flattened 6th to fall back hauntingly to the tonic.
In this case we are in a major key, the flattened 6th rises from A# to B: