In a large bowl or bowl of a stand mixer, pour about 1/2 cup of cold water in. Sprinkle all gelatin packets on top and let coagulate for a few minutes.
Meanwhile, in a medium sized pot, bring granulated sugar, corn syrup, salt and remaining water to a boil. Bring mixture to 240° degrees, about 10 minutes of boiling. Use a thermometer to monitor temperature as there is no way to tell from watching as to the temperature.
Once the mixture reaches 240°, remove from heat and carefully pour into bowl with gelatin. Using the whisk attachment, use low speed to incorporate mixture.
Once incorporated, turn to high speed and let mix for about 12 minutes. It should start fluffing up and looking like marshmallow cream at towards the end.
Spray a 9x13 inch pan with non-stick spray and dust generously with part of the powdered sugar.
Using a rubber spatula, transfer marshmallow mixture to prepared pan. Smooth out top or leave a little design. Dust top with remaining powdered sugar. Let marshmallows sit for 3 hours at room temperature. You may cover but it is not necessary.
Cut marshmallows with a chef's knife into large large squares. Marshmallows will be sticky. Dust sides with more powdered sugar to keep from sticking to one another. You may also remove marshmallow in its entirety from pan and use powdered sugar dusted cookie cutter to cut into shapes. Marshmallows can be drizzled with chocolate and sprinkles if desired. Store in an air tight container for up to a week.