Preheat oven to 160 degrees celsius and line two baking trays with greaseproof paper.
Place the butter into a bowl and beat with hand-held beaters or a stand mixer until pale and fluffy.
Sift in the plain flour, icing sugar mixture, cocoa powder and cornflour and fold through with a wooden spoon.
Roll teaspoon-sized balls of the mixture and place onto the prepared trays.
Press down lightly with a fork.
Bake for 10-15 minutes or until just cooked through (do not overcook).
Leave to cool completely on the trays before transferring to a wire rack.
To make the filling, beat the butter, vanilla essence and lemon juice until smooth and creamy.
Slowly add the sifted icing sugar, beating well in between additions.
When the mixture is completely combined and smooth, add one drop of green food colouring and 2 drops of peppermint extract and beat until the colour is evenly dispersed.
Join two biscuits together with 1tsp of filling.
Store in an airtight container.