Place lemon zest and sugar in a bowl and rub the 2 together to release the lemon flavor. Whisk in your eggs and lemon juice.
Place mixture into a double boiler and cook on low. The mixture will become thick as you whisk it throughout the entire cooking process. Once it has thickened and becomes darker cook it for another 5 solid minutes. Whisk the mixture continuously.
Strain the lemon zest out of the lemon cream by using a strainer or colander. Push the cream mixture through the strainer using the back of a spoon or a spatula.
Whisk in cubed butter and mix until smooth.
Cover and chill for 3-4 hours in the fridge.
Remove the mixture from the fridge and mix it until it's softened up and smooth. Fill the tart shell with the lemon cream and chill it for 45 min. It's ready to serve, or store it in the fridge for up to 3 days.