Coat a 9×13 glass or ceramic baking dish with nonstick spray.
In a large bowl, combine the gelatin and 1/2 cup cool water. Let soak.
In a small, deep saucepan, combine the sugar, corn syrup, salt, and 1/2 cup cool water. Cook the mixture over medium heat, stirring, until the sugar completely dissolves. Raise the heat to high and bring to a rapid boil. Allow to boil, without stirring, for 1 minute. Remove from the heat.
Using a handheld mixer (or standing mixer) on low speed, slowly pour the sugar syrup into the softened gelatin. Increase the speed to high, and whip until the mixture is very thick and fluffy, and has cooled to lukewarm, about 10 minutes. Add in the vanilla and beat to incorporate.
Pour the marshmallow mixture into the baking dish and spread evenly. Rap the dish against the counter a couple times. This will help with bubbles. Sprinkle 1/4 cup powdered sugar over the top, and place in the refrigerator, uncovered, for at least 3 hours (or overnight) before cutting.
Use a greased cookie cutter to make heart shapes (or any shape you’d like) and gently separate. Place heart shaped marshmallows in a bowl with 1/4 cup powdered sugar and gently toss to coat them on all sides. Shake off any excess.
You can store the marshmallows in an airtight container, at cool room temperature, for up to 5 days.