Preheat the oven to 180°C/350°F (160°C fan) and grease a 24cm x 17cm x 4cm baking dish with butter (or one of similar proportions).
Add the sugar to the Thermomix bowl and pulverise for 10 sec/speed 9. Add the butter and chop 5 sec/speed 8. Scrape down the sides of the mixing bowl with a spatula. Insert the butterfly whisk and whisk for 2 min/speed 3. Scrape down the sides of the mixing bowl with a spatula.
With the blades rotating on speed 2, add the egg yolks one at a time through the hole in the lid until combined. Scrape down the bowl and mix for 5 sec/speed 2 until thoroughly combined.
Remove the butterfly whisk. Add the milk, lemon juice, lemon zest, and flour to the mixing bowl. Mix for 10 sec/speed 5. Scrape down the bowl and mix again for 10 sec/speed 5 until smooth and lump-free. Pour the batter into a large mixing bowl. Clean and dry the Thermomix bowl and butterfly whisk.
Insert the butterfly whisk. Add the egg whites and whip for 2 min/speed 3.5 until soft peaks form (without the measuring cup). Tip the whipped egg whites into the batter bowl and gently fold them into the batter using a hand-held manual whisk. Swirl it around until thoroughly combined (see photo in recipe post). Don't over-mix.
Pour the pudding mixture into the prepared baking dish and place the dish inside a large roasting pan. Carefully pour boiling water into the roasting pan (not into the pudding), until it comes about three-quarters of the way up the sides of the pudding dish. Be careful when transferring it to the oven.
Bake in the oven for 40 minutes until lightly golden on top and the sponge is fluffed up. Dust with icing sugar and serve straight away with a dollop of cream on top.