Dust an 8 by 12-inch baking sheet generously with confectioners' sugar.
Place 1/2 cup of cold water and the gelatin in the bowl of an electric mixer with a whisk attachment. Let stand for 30 minutes.
In a small heavy saucepan, combine the remaining water, sugar, corn syrup, salt and vanilla. Cook over medium-low heat, stirring, until sugar has dissolved. If necessary, wash down the sides of the pan with a wet pastry brush to dissolve any sugar crystals clinging to the side of the pan.
Raise the heat to high and cook, without stirring, until a candy thermometer registers 240 degrees F. Remove from the heat and, working quickly, add the egg whites to the bowl of the electric mixer and whip on high speed until stiff peaks form. Reduce speed to low and slowly pour the syrup into the bowl, making sure the stream remains between the whisk and side of the bowl. If you want to color the marshmallows, add the food coloring at this stage, until desired color is reached. Increase the speed to high again and whip until the mixture has at least doubled and becomes thick and fluffy.
Pour the mixture onto the baking sheet and spread out evenly using a spatula. Dust the top with confectioners' sugar and let it sit out overnight, uncovered, to dry. Invert the marshmallow mixture onto a cutting board and, using a hot knife, cut into 1 1/2 to 2-inch squares. Dust with more confectioners' sugar.
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