In a bowl of an electric stand mixer fitted with a whisk, combine gelatin and 1/2 cup of cold water. Let sit while you make the syrup.

 

In a small saucepan, combine sugar, corn syrup, salt, and 1/2 cup water. Cook over medium heat until the sugar dissolves. Raise the heat to high and cook until the syrup reaches 240 degrees on a candy thermometer. Remove from the heat.

 

With the mixer on low speed, slowly pour the sugar syrup into the gelatin. Scrap sides and bottom and set the mixer on high speed and whip until the mixture is very thick, about 15 minutes. Add vanilla and mix thoroughly.

 

With a sieve, generously dust an 8 by 12-inch glass or nonmetal baking dish with confectioners' sugar. Pour the marshmallow mixture into the pan, smooth the top, and dust with more confectioners' sugar. Allow to stand uncovered overnight until it dries out.

Turn the marshmallows onto a board and cut them in squares. Dust them with more confectioners' sugar.

 

Alternatively, transfer the marshmallows in a disposable piping bag and pipe dollops of marshmallows on a generously dusted baking pan. Dust with more confectioners’ sugar and allow to dry uncovered overnight.