Step 1: Make the custard from the mix up with the milk/water until slightly thinner than usual.
Step 2: Mash up the mint leaves with a mortar and pestle.
Step 3: Add the mint leaves and food dye to the runny custard.
Step 4: Whisk thoroughly.
Step 5: Place in the fridge for at least 30 minutes, until almost solid and emerald green.
Step 6: Tip: Pour over a dry sponge cake, and it will soften up the cake a treat.