Put the milk, caster sugar and vanilla extract into a pan, over a medium heat, and bring to the boil. Then set on a low simmer just whilst you do the next steps.
Meanwhile lightly whisk the four eggs, just to combine the whites and yokes. Sift the flour and cornflour into a medium sized glass bowl. Add a small portion of the egg mix and combine using a wodden spoon. You are aiming to get it lump free.
Add the remaining eggs and hand whisk until completley combined.
Add the hot milk whisking as you do. Pour the whole lot back into the pan the milk was in.
Whisk over a medium heat until you have a thick custard. Push through a fine sieve and leave to cool.
Whisk the double cream to soft peaks and fold that into the custard.