I like the PP as well, but it does seem too powerful at first glance. I'd consider doing a couple of the following:

Freezing Action. What is the duration of the stun? Stun is very powerful, and a no-fail stun is pretty potent. Consider making this a daze effect or maybe slow and cannot take CA's until the end of your next turn?

Numbing Frost: 11th level, I presume? Maybe make the -1 effect against any cold effect instead of a blanket -1. In that case, I would see a -2 penalty as appropriate, but not both at the same time.

Cold Mastery: The cold resistance portion is important for this PP and shouldn't be touched. I might even add some rider about changing cold-immune creatures into high cold resist ones with respect to your cold powers. Another automatic stun effect is potent, however. I'd feel better if it were on the first crit each encounter (or just once per encounter max). The -5 to save is too powerful for sure. Make it a -2 at most.

Glacial Blast: Is it normal for an encounter power to have a half-damage miss effect? Otherwise, it's a good cold blast power. I would even consider making it 3d8 and targetting all creatures to balance it out.

Arctic Focus: I think this one's a little too attack-oriented for a utility power. It's also one of two sustain-minor-burst powers you have. It's balanced I think, just a little too much overlap. I'm not sure what alternative you'd want here, except maybe some ice-based shielding effect (just a thought).

Ice Age: "Enemies within the zone take a -2 to saves against the frozen, immobilized, and slowed conditions that are caused by cold effects." Did you mean stunned instead of frozen condition? This one's potent, but it's a daily and it's just a burst 1, so I think it's fine. My only concern is that it's more of the stunning effect powers. If nothing else, a little variety may be in order.

Just some thoughts on these. I really do like it, myself. I'm only guessing at what I would consider "balanced", but it would have to be play-tested to be sure. The stun effects may be no big deal or they may be game-breakers. You'll have to test it out to be sure. I also don't know how these powers will work with cold-based feats. Could be overpowered.