Whisk together flours, sugar and cocoa in a medium sized bowl. Add melted butter, milk and vanilla and mix well. Stir in coconut. Press into the baking pan - you'll need to use your hands.
Bake at 180°C for about 20 minutes or until it is cooked - it will be slightly puffy.
For the marshmallow
Add half the water, sugar and peppermint extract to the bowl of an electric mixer. Beat on high speed for about 4 minutes. While this is going, add the remaining water to a heatproof jug or bowl. Sprinkle the gelatine on top, stir and microwave until the gelatine has dissolved.
Keeping mixer running, add the gelatine mixture in a thin steady stream. Beat for another 8 minutes or until the mixture is thick & fluffy. Add a few drops of green food colouring and mix until combined.
Spoon marshmallow onto the cooled base.
Top with sprinkles, leave until the marshmallow has set, then cut into squares.
This is recipe is halved so feel free to double it if you're feeding a crowd.