Open the packets of the gelatin and pour into your stand mixer bowl.
Add ½ cup of cold water
Add the other ½ cup of cold water to a tall sided sauce pan along with the sugar, corn syrup, and kosher salt. Make sure you use a pot you have a lid for.
Put the pot over medium high heat and slap on the lid. Cook for 3 to 4 minutes.
Remove the lid and carefully add your candy thermometer. The mixture will be very hot, so take your time and practice safety.
Cook until the temperature hits 240 degrees. This should take between 6-8 minutes. Immediately remove the pan from the heat, being careful not to splash the mixture.
Turn your mixer onto it's lowest setting to mix together the gelatin and water. Slowly add the sugar mixture from the pot into the mixer bowl while it's running.
Once you have add all the syrup, give it a few seconds, then turn the mixer up to high speed.
Keep at high speed for 12 to 15 minutes until the mixture is thick and the bowl feels lukewarm, not hot.
While the mixer is working, spray a baking pan with non stick spray. I recommend something in the 13x9 size. Mix the powdered sugar and corn starch in a bowl and dust over top the pan. Save some for the marshmallows themselves.
Add in your extract in the last minute.
As fast as you can pour the mixture from the stand mixer into the pan. You can use a spatula sprayed with oil to help get as much out as you can.
Smooth it out the best you and dust with the powdered sugar/corn starch.
Allow 4-5 hours for marshmallows to cool and firm up.
Dust with more sugar/corn starch and slice into pieces.