Make a foil sling for a 13X9 casserole dish. Push the foil into the corners and up the sides of the dish. Smoothing the foil as much as you can. Spray the foil with vegetable spray.
Whish 1/2 cup water and gelatin together in a bowl and let sit until very firm, for about 5 minutes. Add egg whites to stand mixer fitted with a whisk attachment.
Combine granulated sugar, corn syrup, salt and rest of water to a large saucepan. Bring to a boil over medium-high heat and cook. gently swirling the pan occasionally until sugar is dissolved. The mixture will register at 240' it takes about 6-8 minutes. Off heat, immediately whisk in gelatin mixture.
Work quick, whip egg whites on high speed until soft peaks form. 1-2 minutes. While mixer is running, carefully pour in hot syrup, trying to avoid whisk and bowl as much as possible. Whisk until mixture is very thick and bowl is only slightly warm to the touch. About 10 minutes. Reduce speed to low and add vanilla and strawberries. Slowly increase speed to high and mix until incorporated, about 30 seconds. Scraping bowl as needed.
Add lemon zest and stir. Spread marshmallows to prepared foil and spread evenly with a greased rubber spatula. Let sit at room temperature until firm, about 4 hours.
Whisk confectioners' sugar and cornstarch together in a bowl. Lightly dust top of marshmallows with 2 tablespoons of the sugar mixture. Add the rest of the sugar to a gallon ziploc bag.
Place a cutting board on top of the 13X9 casserole and invert the casserole dish on the cutting board. Peel off the foil sling. Coat a chef's knife with cooking spray.
Cut marshmallows crosswise into 8 strips. Then cut each strip into 6 squares. Separate squares and toss in the sugar mixture in the ziploc bag. Shake until well coated.
Remove marshmallows from bag and place in a colander to shake off the excess. Repeat with the remaining marshmallows. Store in an airtight container for up to 2 weeks if they last that long.