Pre-heat your oven to 140C / 285F (fan off), 120C / 250F (fan on). Place the rack in the middle of your oven.
Combine dry ingredients - Add the caster sugar and flour to a small mixing bowl and whisk to combine. Set aside.
Whisk eggs and cream - To a separate medium sized mixing bowl, add the eggs and egg yolks and whisk lightly. Add the whipping cream and vanilla extract and whisk until well combined. Add the sugar and flour mixture and whisk to combine. Cover with plastic wrap and place the mixture in the fridge for 20 minutes to rest. This is a really important step as it will give the air in the mixture time to rise to the top so you can take it off before baking which will prevent cracking.
Zest your lemons - To a separate medium sized mixing bowl zest your lemons.
Juice your lemons - Juice your lemons and run through a small sieve into a jug to get rid of any lemon pulp and small seeds. Set aside.
Skim bubbles off egg mixture - Use a spoon to carefully skim as much of the bubble mixture off the top of your egg and cream mixture. Discard the bubbles.
Sieve egg mixture - Run the egg mixture through a sieve into the bowl with the lemon zest. Use a spatula to gently mix together.
Finish the filling - Add the lemon juice into the egg mixture (only right before you’re ready to bake) and use the spatula to mix until smooth. You want to mix quickly but gently to prevent the lemon from curdling the cream.
Bake - Pour the filling into the tart shell. If there are any bubbles on top of the filling once poured in you can get rid of these using a kitchen torch. Very slowly and carefully transfer the tart to the middle rack of your oven. Bake for 45-47 minutes. Do not open the oven door until the 40 minute mark to check if it’s done.
How to check your tart is baked - Gently tap the tray with your hand (use an oven mitt if you need) and if there’s a slightly jiggle in the center of the filling, it’s finished baking.
Cool down - Once baked, keep the tart in the oven and allow to cool with the door slightly ajar for an hour. Take out of the oven and bring to room temperature then chill in the fridge for 4 hours or overnight. Slice using a large, sharp knife and serve!