Before you start, line a medium size baking dish or tin with baking paper. Combine icing sugar and cornflour in a small bowl and use a sieve to dust lined tray with half of the mixture. Set aside.
Place cold water measure in a small bowl and sprinkle over gelatine. Set aside.
Combine sugar and second measure of water in a medium pot and bring to a simmer on medium-high heat, stirring occasionally, until sugar dissolves.
Add gelatine mixture to pot, stir and bring to the boil. Boil for 10-12 minutes, until thick and syrupy or a sugar thermometer reaches 120°C.
Carefully transfer sugar mixture to a large heat-proof bowl or bowl of stand mixer. Add pink colouring and beat with electric beater or whisk attachment on high speed for about 8-10 minutes, until thick, fluffy and cooled.
Transfer marshmallow mixture to prepared tray and spread out evenly with a spatula. Dust over remaining icing sugar mixture and set aside at room temperature to set completely for about 1 hour.
When marshmallow has set, use fish cookie cutters dipped in cornflour to cut out fish. Lay each piece on a lined tray, making sure no fish are touching.
Roughly chop chocolate and place in a heat-proof bowl over a pot of simmering water. Heat for about 5 minutes, until chocolate is fully melted.
To finish, using a fork, drop fish one at a time into melted chocolate. Use the fork to remove and shake off excess chocolate. Return to lined tray. Transfer to fridge to set chocolate. Once set, keep in an airtight container in the fridge.