Place gelatin in the bowl of your stand mixer. Add one half cup cool water and mix to dissolve.
Prepare your pan. Cut a parchment sheet to fit one way in pan. Place the thinner parchment in the pan with both sides of the parchment sticking up. Spray the pan with cooking spray and place the other parchment at 90 degrees to the first so that there will be parchment "handles" on both sides of the marshmallows. Spray the parchment with cooking spray.
Place the sugar, the other half of the water and corn syrup in a heavy pan over medium heat. Stir until the sugar dissolves, being careful not to get sugar on the side of the pan. (If you do rinse if off with a little water or a pastry brush dipped in water.)
After the sugar dissolves, turn up the heat to medium high / high and do not stir again. This is important as you do not want the sugar to recrystallize.
Cook until the sugar mixture reaches 240 degrees. Check several times if your thermometer does not stay suspended in your pan. Use your long silicone oven mitts on your hands when you check the temperature in several places in your pan.
When your sugar mixture reaches 240 degrees, turn off the heat and turn your stand mixer to the low setting.
Slowly pour the sugar mixture between the side of the bowl and the beater while wearing the silicone oven mitts. Scrap the pan and turn the mixer to high.
Beat the marshmallows until they turn white and become stiffer. They will be warm, but not hot (about 95 degrees). This will take about 5-8 minutes. Add the vanilla near the end of the mixing.
Quickly transfer the mixture to the prepared pan. Spread if necessary. Let stand for several hours or overnight for marshmallows to firm up.
Sprinkle a cutting board liberally with a mixture of powdered sugar and corn starch.
To cut the marshmallows, carefully pull on both sides of the parchment to release the marshmallows. Take the whole slab of marshmallows out of the pan and put them on the cutting board.
Cut a strip of marshmallows with a knife or pizza wheel.
After cutting a strip of marshmallows, turn that strip so that it is coated in powdered sugar to prevent sticking. Shake off any excess. Add more powdered sugar to your cutting board as necessary. Cut into squares.
Store marshmallows in an airtight container with parchment between layers and space between adjacent marshmallows.
This recipe is from King Arthur Baking Company, Homemade Marshmallows.*You can also use 3 packets of gelatin.**You may want. to add additional starch to the confectioners sugar (corn, tapioca etc)