In a large mixing bowl, mix ½ cup of cold water with the gelatin powder. Let it sit and bloom while you prepare the sugar syrup.
In a saucepan, combinelight corn syrup, granulated sugar, salt, and ½ cup of cold water. Heat the mixture over medium high heat, stirring constantly, until the sugar is dissolved. Once dissolved, stop stirring, lower the heat to medium, and insert a candy thermometer into the mixture.
Boil the syrup until it reaches 240°F (115°C). This might take about 15 minutes.
Whisk the bloomed gelatin into small pieces. Gradually pour the hot sugar syrup into the bloomed gelatin while whisking continuously until the two mixtures are completely mixed.
Using a stand mixer or hand mixer, beat the mixture on medium-high speed for about 15 minutes until it becomes thick and white.
Pour the marshmallow mixture into the prepared baking pan, then add your preferred toppings over the surface. Allow the marshmallow to sit at room temperature, uncovered, for at least 4 hours or overnight until it firms up.
Dust your hands and a cutting board with cornstarch. Carefully remove the marshmallow from the pan and place it onto the cutting board with the decorated side facing up. Brush a sharp knife with vegetable oil, then dust it with cornstarch, cut the marshmallow block into 6 or more pieces.
Toss the cut marshmallows in cornstarch to prevent sticking. Store them in an airtight container at cool room temperature for 2 to 4 weeks.