Prepare the pan. In a small bowl, combine the confectioners' sugar and cornstarch. Lightly spray a 13 x 9-inch baking pan with cooking spray. Sprinkle the sugar and cornstarch mixture all over, till it covers the pan completely. Make sure to cover the bottom and sides of the pan. If there is any remaining sugar-cornstarch mixture, put aside for use later.
To make the marshmallows: Meanwhile, place the contents of the gelatin envelopes into the bowl of the stand mixer. Add 1/2 cup of the cold water. Make sure the whisk attachment of the mixer is close by.
Separately, prepare the syrup. In a small saucepan, combine the remaining 1/2 cup water, granulated white sugar, corn syrup and salt. Cook on medium high heat, stirring to make sure no sugar burns to the bottom. Using a candy thermometer, clip it onto the side of the pan and continue to cook till the syrup mixture reads 240 degrees. This should take about 8 to 10 minutes. If you don't have a candy thermometer, you can use a meat thermometer, but do not let it lean on the pot or it will affect your reading. Once the mixture is this desired temperature, immediately remove the syrup from the stove top. Slowly pour the sugar syrup into the gelatin mixture waiting in the mixer bowl. Pour the syrup from the sides of the bowl. Work quickly and stir this mixture with a spoon to make the gelatin dissolve from its gelled state into the liquid sugar.
Fit the whisk attachment on the stand mixer. To mix the gelatin-syrup mixture, allow the mixer to progress from a slow speed gradually increasing the speed to high. Continue to mix and whip till the mixture becomes very thick and feels lukewarm to the touch. This will take about 12 to 15 minutes. Add the vanilla extract at the last minute of whipping. You will notice the syrup-gelatin mixture will transform to a white, fluffy, elevated puffy mixture. Pour this mixture into the prepared pan lined with confectioners' sugar and cornstarch. Use a pre-oiled spatula to spread the white marshmallow mixture all over the pan. Using the remaining confectioners' sugar-cornstarch mixture left from the pan lining, lightly dust the top, till most of it is covered. Allow the marshmallow mixture in the pan to sit uncovered for at least 4 hours or up to overnight.
To prepare for serving: After 4 hours or the next day, turn the marshmallow sheet out on the cutting board. If difficult to move, lift the corners with a well-oiled spatula and pull it out with your hands.
To cut: Using a pre-oiled, pre-dusted with confectioner's sugar pizza cutter, cut the sheet into strips. Then cut them into cubes measuring 2 inches. Once cut, dust all sides with remaining powdered sugar-cornstarch mixture, adding more if needed. Transfer the mallows on a rack, where you can add the chocolate.
To make the frosting for marshmallows : Cut up 2 whole chocolate candy bars into chunks. I used a combination of regular milk chocolate and the dark type.Microwave chocolate chunks on high for about 2 to 3 minutes. Repeat for 30 seconds more if not yet melted.
Arrange bare marshmallow pieces on a rack, with a tray underneath it.
As soon as the chocolate bars have melted, pour them over the marshmallows. Do this while chocolate is still hot.
Have a piece of parchment or wax paper ready under the rack to catch the dripping chocolate.
Freeze or refrigerate the choco-covered marshmallows, so that chocolate remains firm.
Cook's comments: If you've never done homemade marshmallows from scratch before, here is a helpful video from my good friend Chef Jenni Field on how to do it. Though her recipe and ingredients slightly differ from mine, you'll see first hand how to make them. For other delightful recipes and baking tips, check out Jenni's site Pastry Chef Online, too.