Line a 9×13-inch baking dish with parchment paper and spray with nonstick cooking spray.
To the bowl of a stand mixer, add 1 cup of cold water. Place the bowl on the mixer with the whisk attachment. Pour the gelatin into the water and whisk together. Let it rest for a few minutes to allow the gelatin to hydrate and bloom.
Meanwhile, in a large, heavy bottom pot over high heat, pour in the remaining cold water, sugar, and corn syrup. Cook, stirring constantly until all of the sugar has dissolved. Once the sugar has dissolved, stop stirring and cook until the mixture reaches a temperature of 252°F on a candy thermometer.
Remove the pot from the heat. Let the mixture cool to 212°F.
Once cooled to 212°F, carefully pour the sugar mixture into the gelatin mixture, whisking on low speed for about 30 seconds, or until combined. When combined, increase the speed to high. Continue to whisk on high for 11-13 minutes, or until the mixture is very thick (doubled in size).
Mix in the vanilla.
Spray your tools with nonstick spray. Then, pour the marshmallow mixture into the prepared dish, spreading it out evenly.
Pour the strawberry jam over the top. Using a knife, marble the jam into the marshmallows. Pass the knife through the marshmallows and strawberries going in both directions, making sure they are very well incorporated.
Let the marshmallows sit out for at least 4 hours, up to overnight, until set and firm.
When the marshmallows are ready, mix the confectioners' sugar with the cornstarch in a small bowl (optional). Dust a pizza cutter with the mixture and cut the marshmallows into your preferred sizes.
Once cut, dredge each cut side of the marshmallows in the confectioners' sugar mixture.
Store in an airtight container at room temperature for up to 3-5 days