You will need:
2 1/2 tablespoons unflavored gelatin
1 1/2 cup granulated sugar
1 cup light corn syrup
1/4 teaspoon salt
2 tablespoons pure vanilla extract
Confectioners' sugar (for dusting)
Combine gelatin and 1/2 cup cold water in the bowl of an electric mixer with whisk attachment. Let it stand 30 minutes.
Combine granulated sugar, corn syrup, salt and 1/2 cup of water in a small heavy saucepan; place over low heat and stir until sugar has dissolved. Wash down sides of pan with a wet pastry brush to dissolve sugar crystals.
Clip on a candy thermometer; raise heat to high. Cook syrup without stirring until it reaches 244°F (firm-ball stage). Immediately remove pan from heat.
With mixer on low speed, slowly and carefully pour syrup into the softened gelatin. Increase speed to high; beat until mixture is very thick and white and has almost tripled in volume, about 15 minutes. Add vanilla; beat to incorporate.
Generously dust an 8x12" glass baking pan with confectioners' sugar. Pour marshmallow mixture into pan. Dust with confectioners' sugar; let stand overnight, uncovered, to dry out. Recipe makes approximately 40 marshmallows.
Note: Add a few drops of food coloring for colorful seasonal treats.