Separate the yolks from the whites. You can freeze the egg whites for later or make a Swiss meringue buttercream with them!
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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.
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Juice the lemons until you have about half a cup of lemon juice. Don't forget to strain out the seeds.
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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.
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Heat on low, while stirring constantly with a wire whisk until mixture thickens, just starts to bubble and coats back of wooden spoon.
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Remove pot from heat, then add the cold, cubed butter and mix until melted.
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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.
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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.