Grease an 8x8 inch baking dish. Lightly grease the pan with softened butter.
Dissolve the 3 packets unflavored gelatin in 1/2 cup of water in the bowl of your stand mixer.
In a saucepan cook the sugar, salt, corn syrup and 1/2 cup water, until it reads 245°F on a candy thermometer. Turn the mixer on low-speed and carefully pour in the hot sugar syrup along the side of the bowl, being careful that it doesn't hit the whip attachment.
Once you have all the syrup in the bowl, turn the speed up to high and let it mix for about 10 to 12 minutes. The marshmallow will be light, fluffy and cooled to nearly room temperature. This is when you add the vanilla, and any additional flavors or color, if using.
Pour the marshmallow into the pan and allow to sit for several hours or overnight at room temperature. To cut, dust the marshmallows and work surface generously with powdered sugar. Butter the blade of a knife and cut into desired shape. Dust with more sugar to prevent from sticking.
The marshmallows will hold their shape and can be stored in an airtight container for a week. You want to make sure they are well coated with powdered sugar or they will stick together.