In a medium sized sauce pan combine the sugar and the lemon zest. Using a whisk, stir to fully combine the sugar and lemon zest. Add the lemon juice, butter, egg, and egg yolks. Whisk lemon curd until all ingredients are fully incorporated.
Over medium – low heat, stirring consistently with the whisk, bring the mixture to a boil. It is very important that you do not stop whisking/stirring during this time. If you do, the eggs could separate, and you might end up with scrambled eggs inside lemon juice.
Once the lemon curd is boiling STOP WHISKING. Let the lemon curd boil untouched for 8 minutes. You will notice the mixture getting thicker the longer it boils. The mixture will also thicken as it cools.
After 8 minutes of uninterrupted boiling, remove the mixture from the heat and let it cool slightly before pouring it over your cheesecake layer.
Very carefully and gently pour all of the lemon curd mixture over the top of your cheesecake.
Chill for a minimum of 2 hours before moving onto the next steps.