1 cup water
4 packages powdered gelatin
1 tsp vanilla
Some small quantity peppermint extract*
Bring to 235-238F:
2 cups sugar
2 tblspns corn syrup
Enough water to moisten the sugar
Once sugar syrup has reached the appropriate temperature, pour immediately into bowl of kitchenaid (fitted with the paddle attachment, not the whisk!) and turn on to slow (just below whatever speed starts splattering the stuff outside the bowl). As it cools, turn up until it's on high. Once it's safely on high, spray pans (2 8*8 pans or one big cookie sheet or whatever) with cooking spray. Wait until marshmallow is exceedingly fluffy (it came up to the top of my kitchenaid paddle attachment!) and starting to hold its shape decently, like almost stiffly beaten egg whites. Stop the mixer, pull out the paddle attachment quickly, and quickly pour/scrape the stuff into your prepared pans. If you're piping it, there's a very small window to work with before it starts to set and stops piping attractively, so I'd suggest doing half batches and (*gasp*!) not doing anything too intricate.
Allow to cool and set completely. Turn out onto cutting board thoroughly dusted with whatever you're going to use to make them not stick to everything in sight (I generally use a 2:1 mixture of potato starch and powdered sugar), cut into squares or whatever other shape you'd like, and dust all sides with your powdered whatever. Store reasonably tightly covered (if you want them to stay moist) or not covered at all, in a dry environment (if you want them to get slightly tough and dry around the sides so you can toast them!).
* I used 1/8 tsp of legacy peppermint extract (only 1/8 tsp, because I was remembering that it was very strong); unfortunately, it was indeed so ancient that it had lost *all* its alcohol and was only oil. And peppermint oil is strong. If you have this alarming substance, only use a drop or two, as these marshmallows came out Far Too Minty, even with only 1/8 tsp. If you have regular peppermint extract, use your best judgment, but perhaps start with 1/8-1/4 tsp?