Grease a 20x20cm square tin with cooking spray and line base and sides with baking paper allowing some overhang.
Place biscuits into a food processor (reserving 4 mint slice biscuits for later) and blitz until crumbed. Add melted butter and process until combined. Press mixture into base of tin and refrigerate for 20 minutes while preparing the filling.
In a small bowl, sprinkle gelatine powder over cold water and whisk with a fork. Let it stand for 1 minute. Melt in the microwave in 20 second bursts until dissolved. Let it stand to cool slightly.
In a medium bowl, add the blocks of cream cheese and sugar and beat with an electric mixer until smooth. In another bowl, add the thickened cream and beat until soft peaks form. Add the lightly beaten cream to the cream cheese mixture and beat for another minute. Add the gelatine, peppermint extract and green food colouring and beat until combined.
Roughly chop the reserved mint slice biscuits and stir through the filling. Pour filling into tin and refrigerate overnight or until set.
Garnish with a dusting of cocoa powder and mint leaves.