Add the gelatin and 1/4 cup cold water to a small mixing bowl and mix until well combined. Set aside to bloom (gelatin crystals will soak up the water).
Add the sugar and water to a small saucepan and bring to gentle boil on medium heat. Continue cooking for 5-6 minutes. Then take off the heat and add the gelatin. Gently mix until gelatin is melted.
Add the mixture to the bowl of a stand mixer fitted with a balloon whisk and whisk on medium speed for a couple minutes, then switch to high speed until the mixture becomes thick and glossy. About 7-8 minutes.
Transfer to a piping bag fitted with a small round tip. You’ll need to work quickly here because the marshmallow will eventually set. To make each ghost begin by piping a bulb and then as you swish down stop squeezing the bag to make the little tail. Aim to make the ghost different sizes and have them floating in different directions. Once you’ve covered the top of the brownies, transfer to the fridge for 30 minutes to set.
To finish the little marshmallow ghosts, add the melted chocolate to a piping ag and snip a small bit of the end off and pipe little eyes and mouths on the ghost. Dip a food safe paint brush into some rose lustre dust and draw little rose coloured cheeks on each ghost. Slice to serve.