In a small saucepan, combine the lemon juice, lemon zest & 1/2 cup of sugar and whisk to combine. Set over medium heat, and bring to a boil while whisking occasionally. Once the syrup has reached a boil, remove from the heat and reserve.
In a medium mixing bowl, combine the eggs & 1/4 cup of sugar. Whisk to thoroughly combine. You want the eggs and sugar completely incorporated and almost smooth in texture.
Next, you'll temper the egg mixture. Using a ladle, slowly pour one scoop of the lemon syrup into the egg mixture, while whisking continuously. This action will warm the eggs slowly to avoid getting lemon scrambled eggs.
Continue to pour in the lemon syrup, one ladle full at a time, until 1/3 of the syrup is transferred. Whisk continuously.
After whisking together, pour back into the saucepan and bring to a slow boil on medium heat. Whisking the entire time. The liquid will begin to thicken.
Once at a low boil, keep whisking for 1 minute. The curd will begin to look like a viscous hollandaise sauce.
Transfer the curd to a medium sized mixing bowl and cool for 1 minute.
Slowly mix in the butter, 1 chunk at a time, using a spatula. Completely blend.