1 cup cold water, divided
3 ¼-oz. packets unflavored gelatin
2 cups granulated white sugar
1 cup light corn syrup
¼ teaspoon salt
2 teaspoons vanilla extract (or other flavoring)
Food coloring as desired
3 to 4 cups powdered sugar, sifted, for coating pan and dusting marshmallows
Grease a 9 x 13-inch glass baking dish with unsalted butter and line the bottom with a piece of parchment paper. Generously sift powdered sugar over the paper to coat, about 2 to 3 tablespoons. In the bowl of your electric mixer, add ½ cup cold water and then add the three packets of gelatin. Allow to sit until gelatin softens, about 15 to 20 minutes.
In a medium saucepan, cook the remaining ½ cup of water with the granulated sugar, corn syrup and salt over medium heat, stirring constantly until sugar dissolves and mixture comes to a boil. Cover the pan and boil for about two minutes, then uncover and attach a candy thermometer to the side of the pan.
Adjust the heat to high and bring the mixture to a boil, without stirring, until it reaches 240 degrees, or the “soft ball” stage, about 10 minutes. Remove from heat.
Using the whisk attachment, turn your mixer to its lowest setting and slowly add the hot liquid to the softened gelatin. To avoid hot splatters, gradually increase to high once the syrup has been added. Whip the mixture until very thick and stiff, about ten minutes, then add the vanilla and any food coloring and beat until combined, about a minute. When ready, the mixture will have tripled in volume and the sides of the bowl should be lukewarm.
Use a spatula to scrape the marshmallow cream into the prepared pan and spread into the corners the best you can. At this stage the marshmallow mixture will be quite sticky, so dip the spatula into water if needed. I found it easiest if I just dipped my hands in water and used them to smooth the mixture into the pan.
Dust the marshmallow with another heavy coating of sifted powdered sugar and leave uncovered to set at room temperature for at least 12 hours, or overnight. To remove from pan, run a knife around the edge and then use a spatula to gently lift an edge of the marshmallow.
Use your hands to turn the marshmallow out onto a baking sheet dusted with powdered sugar, paper side up. Remove the parchment paper and dust the top with more powdered sugar. Use a sharp knife, scissors, pizza wheel or cookie cutter to cut the marshmallow into desired shape.
Fill another baking dish or pan with 1 cup of sifted powdered sugar and toss each cut marshallow in to coat, then use a small sieve to shake off any excess sugar. Store in an airtight container at room temperature for up to two weeks.