I've heard and read all kinds of 'recipes' for hardening a frizzen.
What's your favorite method?

I've typically been a kasenite/cherry red subscriber. I recently tried to harden a worn out Lyman frizzen for a friend, but things didn't go well.
I suspect it was my differential heating - I had torch issues mid-process.

I've also heard you really only have one shot and hardening or it's scrap.

I tried Cherry Red a couple times on a TC frizzen, it helps a little bit. Then, an old timer told me to get a good fire going in the fireplace. While the fire is warming up, wrap the frizzen in leather scraps and wad it up in more scraps in a tin can (vegetable or fruit can). Squeeze the top together and roll it over so it’s closed. Shove it into the hot coals and keep the fire going. 60 to 90 minutes, take it out and drop the can in to a tub of cool water. Once it’s cool to the touch, dig out the frizzen. I’ve now done this to 3 different flintlocks and it has helps noticeably on all 3.

0ne and done is a myth.
I have had to treat my Thompson Hawken several times in succession to get it to a acceptable point.
take the frizzen off the lock
lock the pivot in a pair of forceps or vise grips (gently)
mount it in a vise with the feather horizontal.
use your propane or whatever to heat the feather just cherry red.
sprinkle the compound on until it fully covers the face. maintain heat while the compound turns liquid.
remove heat and dunk the feather in very warm water in a figure eight motion.
the clamp on the pivot acts as a heat sink so retempering that isn't necessary.
my process probably gives those trained in metal work the vapors but it works.
i have a replacement frizzen for the TC in a drawer just in case, but 500 shots or so later haven't needed it.

A fact of life for a case hardened frizzen called upon to strike sparks thousands of times is that the hardened surface will eventually wear through and re-hardening must be done. (And always polish the surface smooth before doing it.) Bear in mind those sparks you see aren't bits of flint, they're steel molecules which are violently torn from the mother steel by the even harder flint. That violent tearing action generates heat during that nanosecond, hence the "spark". Hit the steel with a flint enough times and tear off enough molecules and wear becomes evident.

Good sparking is dependent on how hard the steel is. The harder the steel the more resistant it is to parting company with surface molecules, and the more heat (sparks) is generated. One more reason to ignore el-cheapo locks.

I've re-hardened my share of frizzens (mostly back in another lifetime when I lived and breathed this stuff) and always used good old Kasenit. I haven't had reason to use Cherry Red and probably never will because I have a goodly amount of the old formula Kasenit, 50+ year old stuff with lots of cyanide in it - stuff I inherited from my Gramps through my Dad and they've been gone 50 and 30 years respectively now. (The old fella had so much of it that he poured it around the walls of his shop to kill rats and mice, until one day my Dad caught him at it and made him cease and desist!)

It is not just the hardness of the frizzen, but also the carbon content. Higher carbon content gives better sparks ( white hot, branched sparks, vs cooler yellow straight sparks from low carbon frizzens).

The best solution is to find gunsmith who can "resole" or reface the frizzen with high carbon steel. There was a gifted black powder rifle builder up here in Maine who did that work ( Bruce Jones in Waldoboro, Maine). I haven't shot with him in 15+ years. Not sure if he is still with us, but he was "the man" up here for lock work. Maybe Track of the Wolf or Dixon's in Pennsylvania can do it, or knows someone to refer out the work. Similarly with Chambers Locks in NC(?). Jim Chambers may have sold his business, don't know, but he used to modify Siler flintlock actions into precision marvels that were quick sparking. He knows what he is doing with flintlock performance. Cabin Creek??

It's not just hardness, but the % carbon of the frizzen's fence (face).

There used to be aftermarket high carbon drop-in frizzens available for T/C and Lyman flintlocks. Not sure if they are still make them. I bought a replacement high carbon aftermarket frizzen for my T/C Hawken. Man, what an improvement. Go on the traditional flint lock forums and ask around. I've been out of competitive flintlock shooting for 15+ years, and not sure who sells what now. Maybe you can place a "WTB" post on muzzleloading websites or an ad in "Muzzleblasts" magazine (NMLRA magazine) for a high carbon Lyman frizzen, or someone who could resole your current frizzen.

L&R / RPL lock company makes a replacement Lyman flintlock, and sells parts. They have a replacement frizzen for $35. Give them a call and ask if this frizzen is high carbon, and hardened. If not ask if they can do it L&R makes excellent locks. This one is by Investment Arms. I don't know much about them. Might be an option.
https://lr-rpl.com/product/rpl-lyman-investarms-flintlock-parts-05/

A thread to read:
https://www.huntingpa.com/threads/tc-hawken-frizzen-etc.165242/

In my younger days I worked with metalurgist who taught me about carbon migration. There's a few ways to do it, all will be a bit involved.

Placing the part in a chamber covered in bone or wood char and holding it at 1400* (iirc) and then quenching in oil being the best for carbon migration.

That, in a nutshell, is carburizing (case hardening).

How many shots does a typical frizzen last?

I have at least 600-700++ shots on my large Siler flintlock action's frizzen ( Yorktowne rifle by Cabin Creek muzzleloading). Doesn't look any worse from wear. If "moguls" or waves develop on the frizzen, you can dress the face of the frizzen on a belt sander, and start over again. Never had to do that to this Siler's frizzen.

Covering the striking face of a frizzen with a thin piece of high carbon tool steel shouldn't be a terribly difficult job. It could be shaped with a grinder or a belt sander and either TIG welded or even silver soldered around the edges where the flint doesn't hit to attach it. With proper heat treating, this stuff gets as hard as woodpecker lips!

https://www.ebay.com/itm/295992135359?

I've reached out to a buddy for confirmation of where he had his done but I'm thinking it was The Log Cabin Shop in Lodi, Ohio.

https://www.logcabinshop.com/

I've tig welded them using music wire as a filler rod and then grind flat. It's extremely hard, probably above 55 rockwell C and won't wear.

I used to just heat them in a very carbon rich oxy-acetylene flame. I would play this flame on the wear surface until it looked wet (a very bright red), then quench a water soluble oil/ water mix. The hard surface tig weld sounds like a great system. GD