Preheat the oven to 180°C (160°C fan forced). Line 2 x 12-cup muffin pans with green paper cupcake cases.
Open the cake box, remove enclosed frosting sachet and reserve for another use. Place the cake mix, eggs, butter and milk in a bowl, then whisk (by hand or with an electric mixer) for 2-3 minutes until smooth and creamy. Whisk in enough green food colouring drops or gel to give a fairly bright shade of green. Divide the mixture equally among the cupcake cases and bake for 18-22 minutes. Remove from the oven and cool in the pans for 10 minutes, then transfer to a wire rack and leave to cool completely; about 30 minutes.
To make the frosting, beat the vanilla frosting and peppermint essence together in a large bowl. Spoon about half of the frosting into a large disposable piping bag fitted with a plain 1cm nozzle. Insert the tip of the nozzle into the centre of 1 cupcake, about halfway down. Gently squeeze the piping bag, pulling upward until the cupcake swells slightly and the filling comes to the top. Repeat with the remaining cupcakes.
Spoon the remaining frosting into the same piping bag and generously pipe swirls of frosting on top of the centre of each cupcake. Decorate each cupcake with a piece of biscuit to serve. Store any leftover cupcakes in an airtight container in a cool place for 2-3 days.