Separate the yolks from the whites. You can freeze the egg whites for later or make a Swiss meringue buttercream with them!
Zest the lemons. If you have a food processor handy I like to pulse the lemon zest and sugar together to really extract all the oils from them but this step is not a must.
Juice the lemons until you have about half a cup of lemon juice. Don't forget to strain out the seeds.
Strain egg yolks into a pot and beat lightly with a wire whisk. Whisk in the sugar and give it a good mix until lighter in color. Stir in lemon juice gradually.
Heat on low, while stirring constantly with a wire whisk until mixture thickens, just starts to bubble and coats back of wooden spoon.
Remove pot from heat, then add the cold, cubed butter and mix until melted.
This is optional but I like to strain the final mixture to remove any bits of zest, etc. The final curd will be SILKY SMOOTH and amazing.
Transfer the curd to a sealable container or bowl. If string in a bowl make sure to cover the surface with plastic wrap to prevent a skin from forming.
