Special equipment: If making round marshmallows, twelve 2 1/2-inch round pastry molds, acetate strips lightly wiped with butter, disposable pastry bags and a large round pastry tip
To make the marshmallows: Set up a stand mixer with a whisk attachment.
Submerge the gelatin sheets in a large bowl of water. (If you are using the powdered gelatin, dissolve the 3 packets in 1/2 cup of water in the bowl of the stand mixer.) Once the gelatin sheets have softened, squeeze out some of the excess water and place in a small saucepan. Melt the sheet gelatin over low heat, stirring once in a while, until totally melted. (If using the powdered, just leave it in the mixer--there's no need to melt it.) Turn off the heat and leave in the pan until ready to use.
In a second saucepan, cook the granulated sugar, corn syrup and 1/2 cup water until it reads 245 degrees F on a candy thermometer. Turn off the heat. Quickly pour the melted gelatin sheets into the bowl of the stand 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 whisk attachment. Add the salt, turn the speed up to high and let it mix for 10 to 12 minutes. The marshmallow will be light, fluffy and cooled to nearly room temperature. Add the vanilla and flavored extract and food coloring if using and mix just long enough to incorporate and achieve an even color
Line a baking sheet with parchment paper and generously dust with powdered sugar.
Round Marshmallows: To make round marshmallows, I used 2 1/2-inch molds made from PVC pipes. (I had them cut to the size I wanted from a long pipe that you can get at a hardware or home supply store. Or, you can buy pastry 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 free-form marshmallows that I'll talk about in a minute.
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 the prepared forms. Allow the marshmallows to sit for several hours or overnight before unmolding them.
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.)
Square Marshmallows: If you want to make square marshmallows, just line a 9-by-9-inch baking dish with parchment that has been generously dusted with powdered sugar. Pour the marshmallow into the pan and allow to sit for several hours or overnight before unmolding and cutting.
Once the marshmallow has set, turn it out onto a cutting board dusted with powdered sugar. Use a knife to cut into squares. If the marshmallow sticks to the knife, dust it with powdered sugar. Dust the cut sides of the marshmallows with more powdered sugar to keep from sticking.
Free-Form Marshmallows: Simply drop spoonfuls of the marshmallow on the prepared sheet. They will spread out, since there isn't a mold to confine them, but they are still as delicious and a lovely rustic look. Allow to sit for several hours or overnight before using.
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. Make your favorite hot chocolate recipe and float the marshmallows. As they sit in the hot liquid they will begin to melt--that's the best part! As the weather warms up, you can use the marshmallows to make s'mores.