Submerge the gelatin sheets in a large bowl of water. (If you are using the powder gelatin, dissolve the 3 packets in 1/2 cup of water in the bowl of your stand mixer.) Once the gelatin has softened, squeeze some of the excess water out. Place it in a small saucepan. (If using the powdered, just leave it in the bowl of the stand mixer, no need to melt it.) Melt the sheet gelatin over low heat, stirring once in a while, until totally melted. Turn off the heat and leave in the pan until ready to use.
In a second saucepan cook the sugar, corn syrup and 1/2 cup water, until it reads 245°F on a candy thermometer. Turn off the heat and quickly pour the melted gelatin sheets into your mixer. 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, add the salt, turn the speed up to high and let it mix for about 10 to 12 minutes.
Generously dust a baking sheet, lined with parchment, with powdered sugar.
The marshmallow will be light, fluffy and cooled to nearly room temperature. This is when you add the vanilla, extract and food coloring.
Round Marshmallows: To make round marshmallows I used 2 1/2-inch molds. Line the molds with acetate strips that are cut to fit and really lightly wiped with butter, then dusted with more powdered sugar. This requires a bit more effort than the square or freeform marshmallows. Using a pastry bag (I suggest using disposable, since it is very tough to refill the bags when working with sticky marshmallow) fitted with a large round tip. Pipe the marshmallow evenly into 12 prepared forms. Allow the marshmallows to sit for several hours, or overnight before unmolding them.
Square Marshmallows: If you want to make square marshmallows, just lightly grease a baking dish with butter. Pour the marshmallow into the pan and allow to sit for several hours or overnight. 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.
To unmold the set marshmallows: Once the marshmallows are set, simply remove the acetate and roll the marshmallows in powdered sugar. (If they stick to the acetate, dip a paring knife in hot water and run it along the acetate.)
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.