Make the filling by halving and juicing the lemons until you have 100ml of juice and the zest of 1 lemon
Whisk the eggs and sugar and salt together until well combined and the sugar is dissolved
Add the lemon juice, zest, double cream and vanilla extract and whisk gently to combine
Place bowl over a pan of gently simmering water and whisk continously until thickened and the mixture coats the back of a spoon. Around 20 minutes.
Remove from the heat and whisk in the cubed butter, a few cubes at a time until fully incorporated
Pour through a sieve into a jug and let cool slightly before filling a 9-inch tart shell
Chill for 4 hours or until set