Line a 28x18cm slice tin with baking paper - make sure you leave paper hanging over the sides to help you remove the Thermomix Peppermint Slice once it's set.
Place the biscuits into your Thermomix bowl and use the 1 sec Turbo function to crush until they are broken up into approximately 2 - 3cm pieces. Transfer to a separate bowl and set aside until needed.
Without washing the Thermomix bowl, add the butter (cut into cubes), condensed milk and peppermint chocolate which has been broken into squares and cook for 3 minutes, 55 degrees, speed 3. If it's not quite combined at the end of the cooking time (time taken will depend on temperature of the ingredients), cook for a further 1 minute, 50 degrees, speed 3.
Add the biscuits to your Thermomix bowl and mix for 20 seconds, speed 2 REVERSE or until combined.
Pour the mixture into your prepared tin (it will be a little sticky) and use a metal spoon to spread and flatten the surface.
Place the white chocolate melts into a CLEAN Thermomix bowl and cook for 3 minutes, 50 degrees, speed 2 before scraping down the sides and cooking for a further 1 minutes, 50 degrees, speed 2 or until melted.
Pour the melted white chocolate over the top of your Thermomix Peppermint Slice and place it into the fridge for 6 hours (preferably overnight) to set before cutting into small pieces.