Line a 20cm x 20cm square cake tin with baking paper, making sure you leave plenty hanging over the edges to help you remove the slice once it’s set.
Place 250g of plain biscuits into your Thermomix Bowl and crush for 3 seconds, Speed 4. Add the remaining 250g and mix for a further 5 seconds, Speed 4. Note: use your spatula/ wooden spoon to roughly break up any remaining large biscuits. Pour the biscuits into a separate bowl and set aside until needed.
Pour the sweetened condensed milk into your Thermomix Bowl and add the butter (cut into 3cm pieces). Cook for 2 minutes, 60 degrees, Speed 1, or until the butter has completely melted.
Add the biscuits, coconut and cocoa powder to your Thermomix Bowl and mix for 10 seconds, Speed 4, REVERSE, or until the mixture has completely combined.
Pour the mixture into your prepared slice tin and flatten the surface with the back of a metal spoon.
Clean your Thermomix Bowl and make sure it’s completely dry before adding the milk chocolate. Cook for 3 minutes, 50 degrees, Speed 2, or until the chocolate has completely melted.
Pour the melted chocolate over the top of your slice and place into the fridge for 4 hours or until set.