Line an 8"x8" baking tin with non stick paper and set aside.
Crush the biscuits and heat the butter, cocoa, brown sugar and condensed milk in a medium saucepan until the butter and sugar have melted and dissolved.
Take it off the heat and add the vanilla extract and biscuit crumbs.
Line the base of the baking tin with the mixture and place in the fridge while you make the filling.
In a bowl place the icing sugar, melted butter, peppermint extract and two tablespoons of water and mix. Add more water (a tablespoon at a time) until you have a thick but spreadable mixture.
Add in a drop of green food colouring until you have the right green colour you want.
Spread over the base and place in the fridge to set for 30 minutes.
Melt the chocolate and butter together and spread over the peppermint middle. Allow to set at room temperature and slice into pieces when the chocolate is just set to avoid cracking.
Once sliced place in the fridge to fully set. (one - two hours).
Notes
You could use a 9x9 tin for a thinner slice. You can omit the green food colouring. Add more or less peppermint extract to your own taste preferences