Line the baking dish with saran wrap and then use your hands to grease the base and perimeter of the lined dish.
Sift confectioner's sugar and starch in a bowl and whisk to combine. Generously line the bottom and side of the lined baking dish with the sugar starch mixture and reserve the rest for later.
In a heavy bottomed pan, add your water, sugar corn syrup, salt and vanilla. Bring the mixture to a low boil, stirring to dissolve the sugar into the water. Using your candy thermometer, bring the mixture to 240F.
While the hot mixture is coming to temperature, add cold water to the stand mixer bowl. Whip on high speed and while whipping, add in versa whip and gelatin. Whip until stiff peaks form, as if you're making meringue.
When the hot mixture has come to temperature and the whip has formed stiff peaks, whip on low speed and slowly drizzle in the hot mixture. The mixture will start to increase in volume as you whip. Let whip for 5 minutes.
Working quickly, transfer the mixture to the baking dish and then dust to cover with the sugar starch mixture. Let sit for 3 hours or overnight.
Slice into squares in the size of your choice, making sure to dust all cut edges after slicing. You can also use unflavoured floss to cut the marshmallows with more ease as they can be quite sticky to work with.
Store in an airtight container in a cool, dry place for up to 1 week.