Warm the lemon juice and sugar in a saucepan over medium-low heat, until the sugar dissolves.
In a medium bowl, whisk the eggs and egg yolks together until pale in color.
Slowly stream in the warm lemon juice from the saucepan, pouring the juice in one tablespoon at a time, whisking well after each addition.
Pour the entire mixture back into the saucepan and whisk over low heat until the mixture turns thick and pudding-like.
Move the saucepan off the stove and stir in the unsalted butter.
Transfer the lemon curd to a medium bowl and cover with a sheet of plastic wrap, placing the plastic wrap directly onto the lemon curd.
Refrigerate until chilled.