Combine powdered sugar and cornstarch in a bowl.  Generously spray a 9 x 13-inch baking pan with nonstick spray.  Dust heavily with sugar cornstarch mixture, but reserve excess sugar mixture for dusting marshmallows with.  Set pan aside.

In a large bowl, pour 1/2 c. cold water in and sprinkle the 3 packages of unflavored gelatin over the top.  Stir to combine.  Let sit to bloom gelatin.

In a large saucepan set over medium-low heat, combine 1/2 c. cold water, corn syrup, granulated sugar, and salt.  Using a candy thermometer, bring the syrup mixture to a boil.  Use a water-moistened pastry brush to wipe down any crystals that form on the pan sides.  Bring mixture to the soft-ball stage (235-240 degree F).  Remove from the heat and let cool for 5 minutes.

Using a standing or hand-held mixer, turn the mixer on to low and slowly drizzle hot syrup onto bloomed gelatin.  Be careful as temperature will be very hot. Gradually increase the speed to high.  Whip mixture until the texture is white, fluffy and shiny, about 10-15 minutes.  During the last minute or so, whip in vanilla.

Pour mixture into prepared pan.  Smooth the top with a spatula and let marshmallows sit uncovered at room temperature for a minimum of 6 hours, or up to 48 hours.

When ready to cut, generously dust a piece of parchment or wax paper with part of the cornstarch mixture.  Turn marshmallows out on the dusted paper.  Cut marshmallows into 1-inch size cubes using a sharp knife.  If needed, dip knife into hot water and wipe off after each cutting.  Use remaining cornstarch mixture to coat all sides of the cut marshmallows.  Store in a covered container in a single layer, or in between parchment/wax paper, for up to 2 weeks at room temperature.