In a saucepan over low heat, melt the chopped dark chocolate with water, stirring until smooth. Remove from heat and let it cool slightly.

In a separate saucepan, combine caster sugar and agar agar powder. Gradually stir in 120ml of water. Bring to a boil over medium heat, stirring constantly. Once boiling, reduce the heat and simmer for 2 minutes.

Slowly pour the agar agar mixture into the melted chocolate, stirring continuously until well combined. Stir in the vanilla extract.

Pour the chocolate marshmallow mixture into a greased and lined baking dish. Smooth the top with a spatula. Allow it to set at room temperature for at least 2 hours, or until firm.

Once set, remove the marshmallow from the baking dish and cut into squares using a sharp knife. Dust with icing sugar to prevent sticking.

Serve and enjoy your delicious homemade chocolate marshmallows!