Line with parchment, allowing a 2-inch overhang on the long sides.
Put granulated sugar, corn syrup, salt, and 3/4 cup water into a saucepan, and bring to a boil, stirring to dissolve sugar. Cook, without stirring, until the syrup registers 238 degrees on a candy thermometer, about 9 minutes.
Meanwhile, put remaining 3/4 cup water into a mixer bowl. Sprinkle with gelatin, and let soften 5 minutes.
Transfer bowl with gelatin to mixer. With mixer on low speed, beat hot syrup into the gelatin mixture. Gradually raise speed to high, and beat until mixture is very stiff, about 12 minutes. Beat in vanilla. Add food coloring, drop by drop, until desired color is reached. Pour into prepared baking dish, and smooth with an offset spatula. Let stand, uncovered, until firm, about 3 hours.
Sift 1 cup confectioners' sugar onto a work surface, unmold marshmallow on top, and remove parchment. Lightly brush a knife with oil, and then trim edges and cut marshmallow into 1 1/2-inch squares. Sift remaining 1/2 cup confectioners' sugar into a bowl, and roll each marshmallow in the sugar to coat.
Marshmallows can be stored in an airtight container for up to 3 days.
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