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 storing in a bowl make sure to cover the surface with plastic wrap to prevent a skin from forming.
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Notes
The most important tip for flavor is to use fresh lemon juice that's been freshly squeezed. Don't be tempted to use bottled lemon juice of any kind as it'll be far too acidic and will ruin the flavor.
You'll know the curd is ready when it noticeably thickens in consistency and coats the back of a wooden spoon.
Don't stop whisking the curd until it's off of the heat or the eggs will scramble and turn lumpy.
Keep the heat low, don't turn it up to quicken the process or the eggs will scramble.
If you don't have a very low setting on your stove it's best to use a double boiler so you don't overcook the eggs.
Once transferred to a bowl or jar place a piece of plastic wrap directly on top of the curd as it cools so a skin doesn't form.
The lemon curd will thicken as it cools it's really important to cool it completely before using it to fill cakes or it won't be the right consistency.
Cold butter will help cool the curd and set it more quickly.
The curd will set completely in the fridge.
Storage - the lemon curd will keep in the fridge for 1 week or can be frozen for up to 3 months.