For curd: In a small saucepan combine egg yolks, sugar, lemon zest, and lemon juice. Heat over medium heat for 5-7 minutes, be sure to stir mixture almost continuously. Once the mixture becomes thick enough that it sticks to your spoon almost like a syrup remove from heat and stir in one tablespoon of butter at a time until all 4 tablespoons are completely dissolved.
Cover with plastic wrap and refrigerate for 2 hours at least. You can store it for 2 days at the most.
Side note: If you lay the plastic wrap directly against the lemon curd while it sets you won't get that weird skin curds and pudding always get while they set typically. It'll make it easier to mix and spread on your cake later.