Preheat oven to 180 degrees C. Grease baking trays. Stir together the plain flour, wholemeal flour, cocoa powder, milk powder, bicarb soda and salt; set aside.
In a large bowl cream together the butter, vegetable oil, brown sugar and white sugar until smooth. Beat in the eggs one at a time then stir in the vanilla and peppermint essences. Gradually stir in the dry ingredients using a wooden spoon. Finally, fold in the milk chocolate chips. Drop by rounded spoonfuls onto the prepared baking trays.
Bake for 8 to 10 minutes in the preheated oven. Allow biscuits to cool on baking tray for 5 minutes before removing to a wire rack to cool completely.