Preheat oven to 180c. Line a slice tin with baking paper.
Add Weetbix, coconut, flour, brown sugar, vanilla and butter to a large bowl and stir until well combined.
Press the mixture into the slice tin, and bake for 15 minutes.
While the base is cooking, place the cold water and sugar into a stand mixer with a whisk attachment and mix for 3 minutes.
Combine the boiling water and gelatine with a fork, then add to the mixer.
Beat for 8 minutes or until thick and white.
Add food colouring if using and continue to beat until mixed through.
Spread the marshmallow on to the slice base and place in the fridge for an hour or so until set.
Add what ever toppings you like to decorate.
Slice and enjoy!