Preheat oven to 160 degrees (fan-forced). Grease 4 large ramekins (or 6 smaller ramekins) and place into a large, deep baking dish.
Add the butter to the Thermomix bowl and melt on 100 degrees, Speed 1, 3 minutes. Allow to cool slightly.
Add the lemon zest, lemon juice, milk and eggs yolks. Mix on Speed 4 for 20 seconds until pale and frothy.
Add the self-raising flour and the caster sugar. Mix on Speed 4 5 seconds. Scrape down the sides of the bowl and mix for a further 10 seconds.
Pour the mixture into a separate bowl and set aside.
In a clean and dry Thermomix bowl, add the room temperature egg whites and mix on Speed 3.5 for 3-4 minutes or until soft peaks have formed.
Fold ¼ of the egg white mixture through the lemon mixture. Repeat with the remaining egg whites (folding through ¼ at a time).
Gently divide the mixture between the prepared ramekins.
Pour boiling water into the base of the baking dish until it is halfway up the sides of the ramekins (but not allowing any water to get into the ramekins).
Bake for 30 minutes or until the pudding top has risen and set (it will be golden).
Allow to cool slightly before carefully removing the ramekins from the dish.
Sift over icing sugar and serve with vanilla ice-cream.