Line a 28cm x 18cm slice tin with baking paper, making sure to leave paper hanging over the sides.
Place the butter into the Thermomix bowl and melt on 2 minutes, 60 degrees, Speed 3 or until melted.
Add the biscuits and crush for 10 seconds on Speed 8.
Add the sweetened condensed milk, lemon juice and coconut (plus finely grated lemon zest if using) to the bowl and mix on Speed 4 for 15 seconds or until combined. Use your spatula to help scrape the sides of your bowl mid way through mixing.
Pour the mixture into your prepared slice tin and use the back of metal spoon to spread it across the tin.
To make the icing, place the icing sugar into the Thermomix bowl and sift by pressing Turbo for 5-10 seconds. Add the lemon juice and mix for 30 seconds on Speed 4.
Note: If your icing is too thick, add a little extra lemon juice to the mixture until it is a thick but spreadable consistency. If the icing is too runny, add a little more icing sugar and mix again.
Pour the icing over the top of your slice and sprinkle the extra coconut over the top.
Place the slice into the fridge for 6 hours (preferably overnight) to allow it to set.