Line a 9-inch square cake tin with baking paper. Allow enough baking paper to overlap to make it easy to lift the slice out of the tin when finished.
To make the shortbread base
Add all the shortbread ingredients to the Thermomix bowl. Mix for 10 seconds / speed 6 then use the Dough function / 20 seconds.
Tip the shortbread crumbs into the prepared cake tin and firmly press it down. Try to ensure it is even, but don't worry too much about it looking perfect, as you will soon cover it with the lemon topping.
Use a fork to prick holes all over the shortbread.
Bake in the oven for 20 minutes.
For the lemon topping
Add all the ingredients to the Thermomix bowl. Mix for 20 seconds / speed 4.
Set aside while you wait for the shortbread base to cook.
Assembling the slice
Remove the shortbread base from the oven.
Pour over the topping. Be sure to do this while the base is still hot. Do not allow it to cool.
Return to the oven and cook for a further 20 minutes.
Cooling & Cutting the slice
Remove the slice from the oven and allow it to cool in the tin on the countertop for an hour.
Now transfer to the fridge and chill for a minimum of 2 hours.
Use the baking paper to lift the slice out of the tin. Sieve icing sugar over the top and use a sharp knife to cut the slice into 16 equal portions.
Notes
Storage: This slice can be stored in an airtight container on the countertop if your kitchen is fairly cool. Otherwise, store in the fridge. Store for up to 5 days.