Preheat oven to 170 degrees celsius (fan-forced).Line an 18cm x 28cm rectangular slice tin with baking paper and set aside.
Melt the butter in the Thermomix bowl for 3 minutes, Speed 2, 100 degrees.
Add the self raising flour, coconut and brown sugar and mix for 15 seconds, Reverse, Speed 4. Scrape down the sides of the bowl and repeat for a further 15 seconds or until completely combined (use the spatula to help mix).
Press the mixture firmly into the base of the prepared tin and smooth with a spoon. Bake for 20 minutes or until lightly golden. Set aside to cool slightly.
Meanwhile, place the icing sugar into the Thermomix bowl and sift by mixing for 8 seconds, Speed 8. Pour the icing sugar into a separate bowl and set aside.
Melt the copha in the Thermomix bowl for 4 minutes, 100 degrees, Speed 2. Add the sifted icing sugar, milk, peppermint extract and green food colouring. Mix for 10 seconds, Speed 4. Scrape down the sides of the bowl and repeat 2-3 times or until smooth. Note: If the mixture is too thick, add an extra tablespoon of milk.
Spread the peppermint mixture over the top of the slightly warm slice base and place into the fridge to cool completely (approximately 2 hours).
Place the chocolate melts into a clean Thermomix bowl. Grate for 10 seconds, Speed 8. Scrape down the sides of the bowl, add the vegetable oil (or coconut oil) and melt for 4 minutes, 80 degrees, Speed 2.
Pour the chocolate topping over the peppermint layer and chill for 30 minutes or until set. Cut into squares when the chocolate has hardened but not completely set.