In a saucepan, combine the agar agar powder and water. Let it sit for 10 minutes to hydrate.

Place the saucepan over medium heat and bring the mixture to a boil, stirring constantly.

Once boiling, reduce the heat to low and simmer for 5 minutes, stirring occasionally.

Gradually add the caster sugar to the saucepan, stirring until completely dissolved.

Stir in the vanilla extract and salt until well combined.

Remove the saucepan from the heat and let the mixture cool for 10-15 minutes.

In a separate bowl, sift together the icing sugar and cornflour. Grease a square baking dish and dust it generously with the icing sugar and cornflour mixture.

Pour the marshmallow mixture into the prepared baking dish, spreading it out evenly with a spatula.

Leave the marshmallows to set at room temperature for at least 4 hours or overnight.

Once set, use a sharp knife to cut the marshmallows into squares. Dust the cut edges with more icing sugar and cornflour mixture to prevent sticking.

Store the homemade marshmallows in an airtight container at room temperature for up to 1 week.