Start by lining a 9×9-inch square pan with parchment paper, ensuring that the edges extend beyond the sides of the pan. This will make it easier to lift the marshmallows out once they’re set. Grease it with cooking spray.
In the bowl of a stand mixer, pour in 1/2 cup of cold water and sprinkle the gelatin evenly over the surface. Give it a mix and let it sit for about 5-10 minutes. This step is known as "blooming" the gelatin, and it ensures that the gelatin fully absorbs the water.
In a medium saucepan, combine the remaining 1/2 cup of water and granulated sugar. Place the saucepan over medium-high heat and stir gently until the sugar has completely dissolved.
Once the sugar has dissolved, stop stirring and let the mixture boil. Continue cooking the syrup until it reaches between 242°F/ 116°C - 245°F/ 118°C. Use a candy thermometer to monitor the temperature closely. You can place a lid on the saucepan for a minute or two, allowing condensation to form and wash down any sugar crystals clinging to the pan's sides.
Fit the stand mixer with a whisk attachment and turn on low speed. Carefully pour the hot sugar syrup into the bloomed gelatin. Be careful during this step because the syrup is extremely hot. Once all the syrup is added, gradually increase the mixer speed.
Increase the speed of the mixer to high and add the vanilla and salt. Continue whisking for about 7-10 minutes or until the mixture becomes thick, glossy, and triples in volume. The mixture should have a thick, ribbon-like consistency that can be poured out of the bowl, but it will do so slowly.
Pour the marshmallow mixture into the prepared pan and smooth out the top with an offset spatula.
Allow the marshmallows to sit at room temperature, uncovered, for at least 3 hours or until they are firm to the touch. If possible, let them sit overnight for the best results.
Once the marshmallows have set, dust a surface with a mixture of powdered sugar and either cornflour or arrowroot flour. Turn the marshmallows out onto the dusted surface. Use a greased sharp knife or cookie cutter to cut the marshmallows into your desired shapes. Toss each piece in the powdered mixture to ensure all sides are coated and to prevent sticking.
Store the marshmallows in an airtight container at room temperature. They will keep fresh for up to 2 months.