Lightly butter a 9×13? baking pan. Line the bottom with parchment paper, then sprinkle it liberally with about 3 heaping tablespoons of confectioner’s sugar. This will make it easier to lift the marshmallows out of the pan, so spread it in every corner.
Pour 1/2 cup cold water into the bowl of your mixer, fitted with a whisk attachment. Sprinkle gelatin over the water and leave it to soften, about 15 minutes.
Place a heavy saucepan on your stove and pour in 1/2 cup cold water then add the sugar, corn syrup, and salt. Turn on to medium high and bring to a boil. Once it boils, cover it with a lid and allow it to continue to boil for about two minutes undisturbed (this allows sugar crystals on the sides to melt). Remove the lid and attach a candy thermometer. Increase the heat to high and continue to boil without stirring until the syrup reaches soft-ball stage or 240°F (115C). This takes anywhere from 6-10 minutes, so watch it carefully. Remove from the heat.
Now turn the mixer (with the softened gelatin) on low and slowly pour a stream of the hot syrup into the mixing bowl by running it down the inside of the bowl. If you are using a hand-mixer, you will need someone to help you with this step. Once all of the syrup is incorporated, slowly turn the mixer up until it is on high. Continue to beat on high until the mixture is stiff and tripled in volume. It will look like thick marshmallow cream. Add vanilla extract or any other flavoring, mix in a tiny dab of food gel as well at this time and beat to mix thoroughly. Continue to add food gel until the desired color is attained.
Scrape this marshmallow mixture into the prepared pan. Try to dollop it all over so that it will be easier to push and spread. This is a very stick step. Rinse your hands lightly so they are damp and use them to smooth and press the mixture evenly in the pan. Using a sifter, cover the top of the marshmallow with icing sugar, about another 3 tablespoons worth.
Once it has cooled, cover the pan with plastic wrap and foil. Let this mixture stand for about 12 hours or overnight at room temperature.
When you’re ready, remove the plastic wrap and foil. Sprinkle a generous amount of confectioner’s sugar onto a cookie sheet. Dip your knife into the sugar and run it around the sides of the pan to loosen. Then dip your cookie cutter in the sugar to cut your shape or square. Lift and gently push the marshmallow out, then roll and toss in the powdered sugar so all sides are covered. Leave the marshmallows in the sugar as you go. When you’re done, shake the excess sugar off each piece by hand or by tossing it lightly in a sifter. Store in an airtight bag or container at room temperature for up to two weeks.
Makes about 30 small (2 inch/5cm) hearts and 8-10 (1 inch/3cm) tiny hearts.