Preheat the oven to 180°C / 160°C Fan /Gas 4 / 350°F.  

Place the low fat spread / margarine, the sweetener and the zest of 1 lemon into a bowl and using a hand held mixer, whisk until just combined. 

Add the lemon juice of both lemons, vanilla paste, egg yolks, flour and milk one by one, whilst continuing to mix until you have a smooth batter.

Clean your whisk attachment thoroughly.

In a separate clean bowl whisk the egg whites until they produce soft peaks - when you remove the whisk the peaks will just hold their shape for a second or so before collapsing. 

Gently fold the two mixtures together using a metal spoon.

Don't over mix as you want to keep the mixture light and fluffy with as much air left in as possible to get some rise.

Spoon the mixture into an ovenproof dish ( I used one that measured 7 ½ inches square).  

Fill a baking tray half full of hot water and place the ovenproof dish in the centre.

Place in the middle of the oven and bake for 45 - 50 minutes until the top turns golden brown and has set.

There will be a creamy lemony sauce below. 

Top with a light dusting of icing / powdered sugar.