Grease and line a 22cm X 32cm rectangular slice tin with baking paper.
Place the biscuits, coconut and lemon zest into the Thermomix bowl and crush on Speed 7 for 5-10 seconds or until the biscuits resemble fine crumbs. Set the crumbs aside.
Place the butter into the Thermomix bowl and melt on Speed 3, 60 degrees, 2 minutes (you may need to scrape down the sides of the bowl down with a spatula after 1 minute).
Add the sweetened condensed milk and extra lemon juice and mix on Speed 3 for 5 seconds until well combined.
Add the reserved dry biscuit mixture to the Thermomix bowl. Mix all ingredients together on Speed 4, 5 seconds. Scrape down the sides with a spatula and mix again on Speed 4 for an extra 10 seconds.
Firmly press the mixture into the prepared tin and place into the fridge.
Place the icing sugar, butter and extra lemon juice into the Thermomix bowl and mix on Speed 4, 20 seconds. Scrape down the sides of the bowl with a spatula. Mix on Speed 4 for an extra 30 seconds. If the frosting is too thick to spread, add a little more lemon juice and mix again.
Spread the frosting over the slice, sprinkle with the extra coconut and place back into the fridge to set completely.
Cut into slices.
Store in an airtight container in the fridge for up to 1 week.