Frozen Caress wording:
Frozen Caress (Su) Whenever the winter witch casts a touch spell, she can infuse the magic with cold as a swift action. This grants the spell the cold descriptor, and adds 1d4 points of cold damage to the spell’s effect. If the touch spell allows a saving throw, a successful save negates this additional cold damage.
So my question is, can I use Frozen Caress on a spell that already has the cold descriptor? It would be pretty decent to use liquid ice as a focus and have a Ray of Frost at level 1 that deals 1d3+1d4+1 damage.
From just that wording I don't see why not. It grants the spell the cold descriptor, but does not say anything about what kind of spell it can enhance beyond having to be a touch spell.