Add butter, 1/2 cup sugar, lemon zest, and lemon juice to a saucepan and bring to a simmer, stirring occasionally.

In another bowl, combine the egg yolks with the remaining 1/2 cup sugar. Slowly add the hot lemon juice mixture into the egg yolk mixture, whisking continuously. When fully combined, return the mixture to the saucepan and continue to cook over medium heat until thickened and the whisk leaves a trail in the curd.

Remove from heat and pass through a mesh strainer set over a bowl to remove the zest and any egg clumps. Place plastic wrap directly on top of the curd and let set in the fridge for at least 30 minutes before serving.

Store in the fridge for 1-2 weeks. Lemon curd can also be frozen for several month.